We did a pirate cruise last time we were in Cancun. It was fun and the drinks were good. Probably not as good a party as at the resort though!
Dressy for the Al a Carte restaurants is pretty much normal but your choice. There is a dress code as well...mostly aimed at the guys though; long pants and closed toe shoes. No t-shirts or beachwear for anyone though. Besides it is fun and sexy to get dressed up for dinner. Casual at all the other places
A real nice break is a day trip over to Isla Mujeres and rent a golf cart get lost on the island drink and drive eat some good food hit the beach and do some shopping. Boob cruise is a MUST!!!!!