My boys grew up in commercial pools, and didn't realize pools had bottoms, till they were 9 or 10.

They are both competitive (now, Masters) swimmers.

One of the last years we were able to get a park cabin at our preferred (and remote) Florida State park, we took a floating island, and anchored it about 150 yards off shore for several days. It was our 'base', in 20' of water. My boys were 9 and 17 that year.