Hi Terry,
If you want to go to a good snorkeling spot, right in Cozumel there is Chankanaab Park. You can get a cab right at the pier and also rent anything you need there, once at the park. Its very easy to do on your own and its a nice beach day. Not really sure if you'd have enough time to really "see" Cancun or the mainland, may-be some others have already done that. Have a great holiday on Victory :-)
Donnassei/Sealover