Despite the dialogue with Atila in the beginning leading you to think that this quest will reward you with lots of gold, this quest actually only rewards you with a wife to sleep with - nothing more. Keep that in mind when attempting to finish this long but unrewarding quest.
The Bella Isabella
First, go to Bermuda and walk around the settlement. Eventually, a man named Atila will approach you and talk to you. He is a black man with blue and white clothes. You can simply talk to him instead of waiting for him to talk to you if you recognize him.
Follow him to the tavern and once you're near the tavern's exterior, you'll immediately be teleported inside the tavern and speak to him. He'll tell you of grand riches stolen by a guy named Salvatore, and that he's currently residing in San Juan. Your logbook will update, and it'll make it sound like you're after the treasure of Salvatore.
Go to San Juan, enter the moneylender's and exit it, then turn left and turn left into the stretch of road again, and keep walking until you reach the 2nd last house in this stretch of road - it's Isabella's Residence. Once inside, speak with Salvatore and then go upstairs into her bedroom and speak with Isabella herself. When you speak with her, make sure to always pick the first dialogue option - if you pick "We're old friends, me an' him." she'll shout for Salvatore to enter the room. In the next dialogue, if you pick "To be honest, it doesn't do much for me. I've seen similar in an English colony." then you'll lose reputation and she'll call you an uncouth fool. Note that if you do choose those options, the quest won't fail.
Now go to the store, and you'll meet Isabella. After a short conversation, you'll have to give the store owner 1200 gold and your logbook will update. If you do not have any gold, the quest will still proceed as normal and you won't lose any money. Now go to the church and she'll ask you to find her brother, who is a passenger aboard a brig called the "Five Holy Wounds of Christ".
So sail to Cumana and speak to the tavern owner or the townsfolk. If they tell you that he's slipped out of the tavern, exit the tavern and then re-enter it, and then hopefully you'll find him in the tavern. Once he's there, he'll be sitting at the round table so speak to him, then head back to your ship (the moment you exit the tavern, Isabella's brother Miguel de Valdez will spawn and talk to you).
Now go back to San Juan and the moment you moor, Miguel will tell you to visit his sister together so follow him into Isabella's house, then a cutscene will occur. After the cutscene, you can speak to her and she'll tell you to visit her this evening but it's not really necessary except to get the logbook update. Go to the tavern and sleep until evening, then head over to Isabella's house. Outside, you'll see an argument between Miguel and Salvatore, and Miguel will be killed by Salvatore's goons (you can't save him). Salvatore will then try to frame you for Miguel's murder, so kill those goons and make sure to loot her brother's body for the promissory notes, then enter Salvatore's house. Now save before speaking to him, at which point he'll call the officers on you. Kill the officers or run from them - it doesn't matter. What you need to do is escape from the city, get to your ship, and sail to Bermuda. Sometimes, when escaping the city, the forts will fire on you so reload to the save before speaking to Salvatore and try again if required.
At Bermuda, enter and exit the tavern, then look slightly to your left. There's an old brick house with stairs leading to a "Room" (it's Atila's house), so enter it and you'll meet Atila. Optional: If you talk to the tavern owner first, you'll find out that Atila bought a house in the city.
Wait for exactly 2 months (if you're at 2nd October you must be at 2nd December) then enter Atila's house. You'll find him sitting in his chair again. Now save in a separate slot before speaking to him and he'll tell you that Isabella has been captured by Salvatore's thugs and that they're now on their way to Portobelo, and you only have only 15-16 days to catch up with them. So make haste and reach Portobelo because if you fail, your logbook will update and you'll have to tell Atila of your failure, thus ending the quest prematurely.
Save before entering sea mode near Portobelo - you'll immediately engage the Brig Rapturous. Now what you want is to board this ship - if you sink it, you'll have to report back to Atila of your failure. Once you board it and kill the captain, you'll come face to face with Isabella - now 2 things will happen here:
- If you have the promissory notes: You'll show her the notes, "soaked in the blood of Don Miguel but still readable", and she'll realize she's been tricked by her husband. She'll request for you to bring her to her cousin Rosita in Belize, but of course you'll say you want to meet with Salvatore first.
- If you do not have the promissory notes: You'll tell Isabella how you "heard with your own ears" how her brother bravely accused Salvatore of forging promissory notes, but when Isabella asks you to produce the promissory notes, you'll say you don't have them. At this point she'll curse you, refusing to come aboard your ship, instead choosing to "die with the sinking ship", even if you kept the ship. Speak to Atila and your quest will end prematurely.
Head into Portobelo, enter the moneylender's residence then exit it, then turn to your right, and enter the first house you see in front of you - it has a yellow hay top. Once you enter, 2 goons will talk to you, thinking you're the captain of the Rapturous. After they realize their mistake, kill them both and then head for Belize.
Once you enter Belize, enter and exit the governor's residence, then head to the house directly in front of you - it's Rosita Fernandez's Residence. Enter it and speak to Rosita, and you'll place Isabella under her care. When you exit the house, Isabella will speak with you, so talk with her then head for Bermuda again.
When you speak to Atila in Bermuda, he'll tell you that Salvatore went to Belize a couple of days ago. You realize what's happening, so make haste to Belize!
Saving Bella Isabella
Head to Belize and speak with Rosita Fernandez. You'll find out that Isabella and her maid had just disappeared. Go out of the city and you'll see a body in the distance. Approach it and you'll realize that it's the body of Rosita's maid. Turn left now and then turn left again, and you'll reach a cave entrance. Enter it and walk all the way to the end.
You'll encounter a bandit who attacks you immediately upon sight so kill him then talk to Isabella. After the conversation, Salvatore will appear and after a short talk, you'll have to gut him. After gutting him, speak with Isabella and she'll agree to marry you, although she'll also ask you to stop being a privateer. When you loot Salvatore's body, he has some nice items, but they only amount up to 75,000 at most so don't fret if you can't pick up much (it's better to keep the jewelry to give to Isabella as a present anyway).
Now head back to Rosita Fernandez's house and you'll see a cutscene. At the end of the cutscene, speak to Isabella and she'll tell you to meet her in San Juan in one month. Now, it doesn't matter whether you meet her in a month or two (or even a year) but she'll only appear in her house in 1 month. Anyway, after a month, go to her house in San Juan and talk to her. She'll agree to marry you the next morning, so sleep in the tavern until the next morning and head to the Church. If you pester her 4 times in her house before the wedding, she'll kick you out but the wedding will still proceed as per normal.
After entering the church, save before walking up to the altar and the wedding will begin. If it doesn't, exit the church, re-enter, then try again. Anyway a black screen will occur saying "Forty minutes passed, over the course of the service" and then just after you two are married, Salvatore's thugs will appear and try to ruin your happy day. All of the thugs will target you, but one of them will try and kill Isabella, so make sure you either block their path, or kill that thug (he's the one with the black pirate hat) first. Either way, kill them all, then speak with Isabella, who will direct you to speak to the governor. Note: If Isabella dies, head back to Atila and speak with him, and your quest will end prematurely - so don't let her die.
Go and speak with the governor and tell him that Salvatore was grade A scum and that you did the best for the city by ridding the world of his goons. Now head back to Isabella, who'll tell you of 2 principles you'll have to follow - the first being paying a monthly fee and the other being to stop privateering - that is, to not sink any ships or capture any cities. Your character will be so smitten with her that he/she will agree immediately.
In any case, you're at the end of the road here. When you talk to Isabella, there will be 5 dialogue options you can choose.
- Oh, I just wanted to talk... Missed you, and all... - This is where you can ask her "what's been happening with you lately?" Often, she will mention the governor.
- Let's see what shape our family budget is in... - Here you can choose to give her a set amount of gold to pay the monthly bills - 25,000, 50,000, 100,000, or 150,000. Give her more money so you don't have to worry about her on your long voyages.
- Dear, I've brought you a present. - Used to give her jewelry like diamonds and sapphire rings to keep her happy. Sometimes by giving her enough presents, she'll be so happy she'll ask you to come sleep with her.
- Listen, dear. About the family debts... - Allows you to sleep with Isabella. She'll get tired after each encounter though, and you'll need to wait a few days before you can sleep with her again.
- I want to sleep. I have no strength... - Basically allows you to sleep like in an inn. You can wake up in the evening or the next morning.
Isabella can be a high maintenance wife. Not only does she require 50,000 per month, if you speak to her more than 3 times regarding the family budget, she will suddenly get annoyed and refuse to talk to you. At this point, there's nothing much you can do except wait a few days for it to blow over before she'll speak to you again.
If you sink ships and capture cities, she'll give you your "first warning", but after that, there seems to be no penalties.
If you neglect to pay off her debts, after the third warning, she will refuse to talk to you and you'll have to wait around 30 days for it to blow over and she'll talk to you again. You still have to pay off that debt though.
If you go on long voyages and have not seen Isabella for many months, at some point when you speak to her about wanting to sleep with her, she'll warn you not to go off too long without visiting her because "there are many interesting caballeros" hinting she's thinking of cheating on you. After a while, when you return, you might find the door locked and the logbook will update - saying she locked the door because of your extended absence. In this case, talk to Rosita and she'll help smooth relations with her cousin and unlock the door for you again.
Important note: If you sleep with anyone other than her (i.e. waitresses, prostitutes or the madame of bordello) she will refuse to open the door to you. Then if you go to Rosita in Belize she will tell you that your marriage has been absolved because of your adultery, stating that you will never see Isabella again. The quest will be removed from your logbook and transferred to the Completed Quest section.