We just got back from being in Chankanaab last week. Enjoyed it. The lockers are free... just find one with a key in it and it's yours. The snorkel equipment was $6.00 per person. The lagoon is no longer used for snorkeling (if it ever was).... which is fine because the area just off the beach is great for snorkeling. There are two outdoor restaurants there and they aren't really cheap. $2.00 for a Coke and $2.00 for bottled water. Food prices are in alignment with that. If you put your towel or sunbathing stuff on a chair I would think it's fairly safe while you are in the water.
We booked a dolphin snorkel swim through the Dolphin Discovery website and saved $100 by doing this ourselves instead of with the ship (Carnival Conquest). The taxi ride from the port is $10.00 one way which was weird to me. It's only a 3 kilometer drive with no traffic. NYC is cheaper than that. The shops around the port were enough for us to find some cool things. Our only regret is not getting off our boat earlier to spend more time at the park snorkeling. The dolphin swim was really cool.
We got into the park for free (instead of $10.00) since we signed up for the dolphin swim... and also a free showing of the sea lion show.... kind of more geared towards kids. I believe these are specials for the next few months.
Anyway, it was a nice experience!