The actually being underwater part makes it a little tricky. If the body is going to be thrashing, water is going to be spilling everywhere and the last thing you want is a slip hazard. With that said, I don't think you necessarily need the water. And if you don't use water, save your money and use a person instead.
If you want to use water for sure though, what I'm thinking is use a static prop instead, but don't have it laying down in the water. Have you seen the original Halloween II? Michael Myers kills someone in a hot tub sort of thing but holding their head under, but their body was out, so when they died the body was limp hanging over the tub with just their head under water. Maybe you could do that and dye the water red with blood sprayed on the back wall to make it look like their throat was slit. I'm not big into gore, but if you're doing a scene like that, that's what I would do.
It's hard to give suggestions without knowing a lot of criteria - like budget and the feel you want in the scene... So that's what I've got!
Twisted Realms Haunted House