No floating!
At least, not in the ray pool.
This is to protect both you and the rays. Stingrays like to bury themselves in the sand (to the point of being basically invisible) and accidentally stepping on one can cause it serious harm. And, as their natural reaction will be to stab at you in self defense with their tail barb (which is trimmed like a fingernail at Sea World for safety, but if it's starting to get long again it can potentially do you harm), there is the slight potential of injury to yourself as well.
So, in the stingray pool, they request that you keep BOTH feet on the ground at all times. Do the 'stingray shuffle,' where you drag your feet along the bottom without lifting either of them off the floor.
By the way, this is also a useful thing to remember if spending any time at the beach in some areas of Florida. Particularly on the Gulf coast, there are lots of stingrays in some shallow areas and the 'stingray shuffle' becomes necessary to protect YOU. =)
In the tropical reef tank, however.. there are a few free-swimming stingrays which you may float around with to your hearts content!
=)
