As everyone is advising, don't miss Ho Chan and Shark & Sting Ray Alley. It's a very short boat ride and your guide will be in the water with you.
As collky says you can snorkel off many docks any time. I've done it many times and the best by far is at the end Ramon's dock. They have established an artifical reef and do not allow fishing. There are 100's of fish under and around the dock of varied species. Of course they are smaller than what you will see at Ho Chan, but still great. I have even see string ray and eel there. There is a ladder to get in and out of the water and boats stay clear, but stay on guard.
Have a great time. It is so wonderfully different than Mexico!! You'll love it!

"Hold on Tight To Your Dreams" ELO