The reason you cant drive your vehicle into the water is because with Arma there is simply no way to get it out. If you where in a cop chase in real life and decided to ditch in the water they could just call in a tow truck or whatever and pull it out, we dont have anything like that here.
As to the no choppers in terror that was added because no matter how good your aim was if there was only 3 deputies on they aren't ever going to stop a chopper and that is exactly the kind of situation we constantly ran into.