You have to go and meet Rueben at the marina bar, but you can't actually get in, as there's no hand symbol on the door. So you're completely stuck basically, although some people with the bug (on xb360 I think) have been able to get round it using some weird mortar glitch (aiming at the door, then pivoting round 180 degrees to get their character to effectively ghost through the doorway).
It's very annoying because I was enjoying the game up until that point for the most part, and now there's absolutely no way of continuing, I tried loading an earlier savegame and it was exactly the same. There's no way I'm gonna load a much older save, slug through an hour or two of play and still have no guarentee that the bug won't still be present.
Ubisoft need to fix it and their complete lack of support or even acknowledgement of such a serious flaw is absolutely dispicable. Yes, it's probably not the easiest bug to fix (since it doesn't happen to everyone, and is cross-platform) but their support staff won't even acknowledge that their product is defective. Even something simple like a noclip cheat would be nice so I could progress with the game.
I've never known a game developer that has let a Sev1 issue like that go outstanding for months, meanwhile issuing silly little enhancements.