Just a reminder for anyone who is stuck, you don't need another user for social chests anymore. They can show up in freemode when your on the ground.
In Jamaica, head towards the castle/fortress. A social chest will randomly turn up outside a house, if not fast travel to a few locations including the location of where you chased the guy at the start.
I know longer have a ps3 since mine died, I'm now on the ps4.
That's difficult to describe. If I search this chests I have my 10 locations where they can be and I visit this locations once every day I play, And after 6 tries I'm, ready.
So my suggestion:
Google ac black flag social chest locations
There should be a link to the steamcommunity. Find the link to the image of social chests and one for a document. Don't try all locations. Take the three in Nassau and one or two in Kingston and some small island, but don't spend so much time. Try it once and come back one hour later. There should be no problems.