There is a recent thread about this already.
http://www.revscene.net/forums/69811...rt-mexico.html
The water museum in Cancun sucks. It is not worth it. It just statues that people just dropped into the ocean. Also its so deep that you cant even reach it even if you hold your breath with the snorkling gear. The water is quite clear though.
edit * The water museum I went to was near Isla Mujeres ( the Island of Women ) which is about a 30min boat ride from Cancun. Maybe its a different water museum that you are interested in. might be better.