To be honest I think that for water in the game it should have sinking physics to the car so it sinks slowly rather than just hitting the water and sinking
It isn't that easy to just modify a physics engine. While this may happen in the future, I don't believe it's a concern as of right now.
You don't need to modify the physics engine to do this, you only need to add proper fluid dynamics to it. Which is also super hard though.
