Yes, I learned a long time before in physics that you call that density (mass/volume). I did't test to give the water physic properties. Theoretically, if I give the water a mass, maybe the engine count the density(?). But I think that's not the only problem. I also noticed very strange turbulance forces (I gave the water the turbulence texture flag) and my balls (the ones in the game, hehehe) played crazy.

PS: @Marco Grubert: Gratulation, the physic engine is a real great and phantastic stuff!!