It would be nice, but it would never happen. I can just see it now...well, my child is almost trained but not quite, and he/she seldom has any accidents, so I think we'll be safe at the pools...then, that child has an 'accident'. It would just lead to more lying in order for parents to do what they wanted to do in the first place.
It would be nice if those that are not potty trained, completely, would just use the kids splash and play areas though. I've been in a pool several times when it had to be cleaned due to an 'accident'. Man, it's a pain. That pool can be out of action for quite a bit of time...time that most wanted to be in the pool.
So, sure it would be nice if kids had to be potty trained. But, it won't happen. I can see the 'swim diaper' police now.....the rule would be no swim diapers in the pool...and guests would be checking kids' bottoms to see how 'padded' they are.