<b>@Goofynut</b>
The water is about 4 to 4.5 feet deep over much of the "standing" area at Stingray City. The boats park in deeper water around the perimeter, so you may have a "short" swim to get where you can stand.
All of the operators I saw there issued some sort of flotation device. Some used diving BC's, some ski jackets, and some Coast Guard style life preservers. If you are concerned about what you will get you could ask your operator, or bring your own.
We also brought cheap (under $3 = "disposable") blow-up pool rafts for our kids. It was easy to have the kids lay across this and look down with their masks, particularly in deeper water (Coral Gardens Smith's Cove, Dzul Ha, etc.). This way they were together, confortable, safe and very manageable - just drag the raft with them on it over the best spots. When we were done, we gave the rafts away to the locals so we didn't have to transport back to the ship.