I drew up plans to have a body floating inside a fish tank that was in dark green watter. The drawing had a safe tank for the mechanics submerged inside. Not sure if that would be what your looking for but if it is let me know I will take a picture of the drawning and post it.
Also, I have a drawing (drawing are all in my black book) of an actor inside a tub covered with soap-suds. She/he was not actually laying down, but crouching instead. When people were to walk past, the actor would stand up for the scare. -There should be fake legs and maybe feet showing above the suds so no one know the actor can stand. The suds are on top of either a 4 inch tank of water (inside the tub), where the actor can hide under, or the suds can just be fake on a sheet of plywood. This might be better, as when the actor stands up, they can easily stand on the plywood for extra hieght.
Hope this triggers your imagination some.