Hahaha! When I read that this hasn't happened to you or Saltypaws I feel off the chair laughing! What is wrong with my game! And this was NOT part of my story! Actually the story wasn't even going to be like this at all! But when Eve drown I was shocked and had to re-plan my WHOLE story! Eve and Grace were going to be HUGE contributors and well when they are dead they really can't...
It may be because some parts of the pool are blocked with the rocks so they can't go out (you know how sims stump their feets and can't reach something on ground, same could happen in the pool). Beside that, if they are tired or fatiqued they will drown but you'll have an hour to get them out (yelling and waving as a signal) and of course very red mood.
I'm loving this story and reading it regularly, very creative and unique set up!