Prison Profits:
When you arrive to Port Royal with a fellow on your ship who suppose to meet with Morgan you are thrown to jail immediately. After the short cut scene with Morgan you are free to go but not so fast. There are numerous cells all along to the exit on both sides so go ahead and check them. Some of them will be empty, others got prisoners lying around(can't talk to them) and some prisoners are just babbling around. However one of them could start a random quest with the above title.
Well the thing is he is either asking you to deliver a letter to his comrades who are apparently holed up in town(you got 2 days for this) or he wants you to free him. Now the first case is simple: you either agree or not, but the second is much more complicated: you got 3 options:
-decline his offer(which is the best thing to do if you want to save yourself a lot of time and reloading and.... oh well just let that scumbag rot you'll find out soon why)
-accept it and try to cut and shoot your way out of the fort(the prisoner can't fight and will go down from one or two swing easily)
-accept it and try to bail him out(you go to the prison guards boss and try to bail him out.I guess its success rate is random if he committed serious crime like burglary or murder your plea will be useless and I wasn't able to free him this way)
If you planning to cut through the fort then good luck it'll take a lot of effort not get yourself and him killed and your best bet will be to moor your ship at the lighthouse before you attempt it. I only managed to free him by running away from the fort with him right behind me but still hard.Now I forgot to mention that his part of the bargain is his secret stash in a cave usually in the Cayman Island which he offers you 50-50. So get him to your ship in one piece try to loose the pursuing ships(since your rep with England will plummet every ship in the vicinity of the lighthouse will attack you)and head for your reward.Once you reach the cave you'll notice he will run towards you to tell you the great news: his bloody stash has been found by someone else

(or more likely there was no stash at all).What do you do now?
Yo man legs it so turn around calmly, pull out your pistol and shoot the bastard in the back
Congratulations you've done it and by now you probably got a few thousand piasters bounty on your head
Now if he asks you to find his pals instead its way easier and more fruitful(is it?).
All you gotta do is run back to Port Royal and look for the house just on the corner where Morgan's house is.Go inside don't mind the house's owner head upstairs and into the room. If everything's fine you should be approached by two handsome robbers and after a short conversation you either give them the letter and they give you a small reward-like 2k-or they decide to cut you into pieces just for fun, so defend yourself and kill them cheerful chaps.
Either way your logbook will be updated so that's it you've done it whooo-hoooo!
I can't stress it enough it is a random quest so it may not happen at all so make sure you save before you thrown into jail.
Good luck.
Oh one more thing! There's a major bug in this quest-in case you are given the later task just make sure you save right after you talk to the prisoner because when you enter PR the two bandit may not spawn in the house upstairs! If that happens just reload as many times as needed, they will be there.
Additional note: It seems this quest can be randomly obtained in every prison or during any event when you are thrown to jail i.e. french gg mission delivering letter to curacao.
Last night I went to Havana prison to find Rock the Brazilian(I know hes in Santiago now) and got the same quest.I promised the prisoner nothing but to speak with the jailor and guess what.
His crime was minor thievery so I could release him for 3k then we sailed to Dominica went into the grotto and voila: his stash was there 206k piasters.
Whoo-hoo! Happy days!