You can buy small waterproof cases that will fit in you trunks or around your neck for your I.D. and cash; anything else, you can usually just ask someone to keep an eye on it for you--no lockers. I have never heard of anyone losing anything on an excursion.
Some places do have lockers you can rent. You didn't say where you were going snorkeling, so I can't answer specifically. Like f-mattox stated, you can purchase the water proof pouches at any local dive shop or on board. Best wishes!
If you're going to Xel-ha, they may have lockers by now. They didn't when we were there, but we were told they were still adding ammenities. Chankanaab Park does not have lockers. If you're going on a catamaran or dive boat, the crew will watch your stuff.