If you like a party atmosphere, you've got to go to Carlos 'n Charlies in Cozumel. Its a blast. You can eat and drink if you'ld like. Everyone with you will be allowed in...they don't card anyone and I think the drinking age is 18 anyways.
If you want to go snorkling in the Cayman's try going to www.baysidewatersports.com
They are helping us plan our wedding (we are doing it at Stingray City) and they have some great deals. A 1/2 day package takes you to Stingray City (which you can't miss) and two other snorkling sites. Its not very expensive either.
If you end up booking, send an email to Jacki and let her know that Joy Englund referred you there. They have the best deals that I've found online.