Would it be possible to hack an object in whatever physics engine you’re using (ie joode or odejava) to somewhat behave like water by just adjusting the surface parameters, like softness and erp, or would it require a much more realistic model that takes the volume of penetration instead of just depth into account?
The question is impossible to answer for you without any additional information. Surely you can have something that resembles water in most physics engines, but if that’s good enough depends entirely on the application. For example in a racing game you usually want very realistic (or at least specialized) physics which most physics engines know of don’t have. If you wer working on a strategy game, I’d be wondering why you’d even bother with an advanced physics engine like ODE.
It is for a fps game that will eventually have ragdoll physics, so having dead bodies interact fairly accurately in water would be a plus.