Hello my friend, first of all thank you for contacting me.
I also had this problem with cheetah vehicle he collided with the water and flip, when changed the handling I do not know the exact if the problem is related to steam version I tested as a non-steam version and the collision does not exist. The physics engine and everything is applied when the vehicle is in water if you want use the car on the water It is unlikely that this work change the traction multiplier and traction loss and don't forget to make changes on the percent submerged.
Sorry I can not help you in a better way, but I believe that in fact this problem is related to the MTA and a different game base, because when the handling of certain vehicles are changed there is a collision with water eg in las venturas, change the suspension of cheetah for example and try to walk near the pirate ship, water will start coming out of the asphalt and the car getting stuck with the steam version.
I'm trying to find out for sure what is causing this problem but apparently it is problem with mta !
Haha, hellyeah !