above.. below ground makes no difference for us.
An adult always needs to be near, Accidents happen...and kids panic, even adults do
All it takes is a simple fall and a hit head.... can be tragic so quickly...
Or a funny Jump (when no jumping is is allowed etc) Simply Not worth the risk....
So for our house rule was, adult is always present........
I was never allowed to swim alone or with my friends without an adult. My friends were even certified life guards. I am surprised that people allow their kids to swim alone. My kids aren't allowed to swim without an adult at the house. and they won't be.