Well, there are multiple ways to kill sims. Drowning, Fire, Electrocution, Old Age, Starvation.
->Starvation is easy, put them in a room where they can take care of all other needs other than hunger (so you don't get a needy ghost that steals your bed or such).
->Drowning is when your sims are fatigued and can't work out or anything. They need to be in the center of the pool for you to box them into it. Working out and swimming aren't the same thing. Before they get fatigued (check the action slot to see when, stop them an hour before) put them in the pool and let them swim, box them in. When they get fatigued they'll have no way to escape.
->Electrocution is easy if you have the teleportion pad and can just have a sim try and upgrade/tinker with it.
->Fire produces the prettiest ghost in my opinion. The cheapest fireplace and about nine rugs should do it. Make sure there is no door to the burning chamber for the rest of your sims will run to the fire like idiots.
Maybe you should try another method than drowning?
Oh, and your sims do eat at work for starvation.
I like killing my sims and keeping their ghosts too.
