I never had my sims drown in TS3. Only exceptions are a few sims outside the played household, such as Fiona and Molly (the Single Moms) both dying of drowning, Molly shortly after Fiona. (IIRC I was playing an household of River McIrish, Holly Alto, Sandi French, the unborn baby Ursine, and River&Holly's adopted child Clover. They lived in Pescadero.)