Safety is paramount when playing around pools.  Beyond the usual "don't drink and swim" and "no runing" it's important to be careful and use proper sex safety methods.  With pools it's not "shrinkage" it's "slippage" to worry about.  There is not much issue with guys, but gals can get into some serious trouble with underwater sex.  The compression of the water can push the pool water up and into places where it can't be easily removed or cleaned and that can lead to infection and much worse.  Best to play and move to the pool's edge for the major events.