water drag physic patch

This new patch make water drag more realistic. 

It make water drag more realistic with minimal change to the code.

To see the difference, just drop unit into water at some speed and see this difference:
     Original: unit hit sea floor instantly.
     With patch: unit slow down when hit water surface and gradually fall to sea floor.

Another difference is the reaction to sudden impulse. In ZK, make atomic bomber (armcybr) fire on submarine (corsub), the difference is:
     Original: submarine bounce off water. 
      With patch: submarine implode toward epicentre of explosion.