This has happened 3 times now. I'm playing with a family of 4, 2 adults and 2 teens. I was having a pool party in the backyard when all of a sudden the daughter drowned then died. Luckily I didn't sav...
When I first tried the insane asylum challenge I gave them a pool and right away three of the Sims drowned. I restarted from a save and three different ones drowned.
I found it was the pool being too narrow that did it. Well at least, when I widened the pool they stopped drowning.