I love giving my Sims a pool in my game, but I agree it would be nice to have more things for interactions with it. Although, there is more than what you have mentioned to do, for instance @TheGoodOldGamer's ideas and you're right by the way, GT does have group interactions for the pool. I made a club for the patients in my Asylum Challenge that included swimming and diving. They had a lot of fun with it, especially one guy. He loved to dive and every single time, he'd get out naked and then walk around looking all proud shocking all the other patients. lol
Sims can also sit by the pool and read or eat, talk (and I think kiss?) in the pool, swim laps, spash each other (as mentioned), they can teach their children to swim and if their bladder is low, even pee in the pool. o.O Gross, I know, but still it made me laugh the first time it happened in one of my saves. Oh, and if you have Seasons, there's a special day for a Polar Bear swim in the Winter. ;)