They're normally run by 18-30's / Escapades / 2wentys etc mate
Not sure who you're going with, so if you're not with those lot, find an 18-30's hotel (they're all over the place in San An) or find a rep and say you wanna go.
you'll be able to get on it even if you're not with those tour operators so no worries...
But the best boat parties are the ones organised by the workers etc, check The Ship inn in san an and see what you can find...