If you're lucky enough to have your own pool, then why not make up some fun games to play with the kids while in the pool?
Kids do love playing by themselves or with others sometimes, but they also love it when a grown up joins in as well. If you have spare time, a pool and some kids, then make up some fun games to entertain them for a while.
Pool races - my brother and sister love swimming races against one another. They find it thrilling and it gives them a great arm work out too. It'll definitely make them tired once they get out. I get a stopwatch and I call out go and then I press stop once the first person reaches the finish line. There are no prizes, but its always fun to play. The rule is that you cannot cheat and you can't get upset if the other person wins - always remind the kids of this.
Retrieving treasure from beneath the pool - If you've got one of those toys where it sinks to the bottom of the pool, get the kids to race to swim down and retrieve it. Works good if you have multiple toys and see who can get the most. Once again remind them that it is just a game, and no snatching or getting upset is allowed.
Pretend you're on an island - If you have blow up pool toys, get each kid to lie in one and then pretend that they all live on different islands or something fun like that. Imaginary games are always the best, and it doesn't involve competition.
Teach the kids to swim - if you are a good swimmer (unlike me) and want to give your kids some extra assistance outside of swimming lessons, teach them how to swim in your own pool. My mum taught my brother how to swim in our pool until he was about 5 and started doing lessons.
Pool safety quiz - If you and the kids or cousins or siblings are relaxing in the pool one night then play a little game, have a pool safety quiz. Ask them questions like, "What would you do if you wanted to go in the pool?" or "What is the most important thing you do once you finish swimming?". Get them to respond and teach them how to safe around the pool.