I would imagine also then that heat management would also be altered a bit to the cooler side.  In fact, it should be possible for an overheating mech to enter  the water to cool down much faster than in air.  Also, the cooling effects of water should also allow a mech to fire much longer without overheating in the first place...

Mechs can be totally submerged, the cockpits are airtight. How else do they fight in those odd environments?

However, I'm not so sure about the pressure-resistance if the cockpit material. I'm sure it would cave in if the cockpit is more than 30, 50 feet under water. Then again, maybe not, if they can fight in space...

If you're looking for a mech handling model I think Mw3 executed this the best out of the entire series.  Mw4 and Mw2 left me feeling like a guy in a rubber suit.  Mw2 because the cockpit was exceedingly simple and Mw4 because it felt like I was looking out of a helmet visor instead of a view port.

At any rate, you have to include the falling down animations from Mw3.  They were done so well that they would trick my brain into believing the motion, particularly the fall on the back animation.