Insanely enough I actually found killing a sim by drowning was the easiest by using a vampire. Again though, it's not directly but it's quite quick. If you drain their energy need then use the alter emotion skill to something positive like happy or confident it stops them from being too uncomfortable to swim in the ocean (does this happen with a pool too? I'm not sure). Anyway, then it's a simple matter of the swim here together interaction and you have one dead sim.
I am super frustrated with us not being able to directly kill sims. The only way I have found is by adding a rare crystal to one of the relics from jungle adventure and then you can poison another sim. That takes 3 days to work though which kinda makes it less satisfying. The death by emotions one is kinda a joke, the amount of work it takes is crazy and just makes the game not fun.
