If you wade a vehicle too deep you can face the problem of flotation. If the vehicle floats, or even just take sufficient weight off the wheels, you can lose traction and even a weak current can drag you downstream.

One of the advantages of open vehicles, or rusted out ones, is that they let the water in and pretty much prevent the vehicle from floating. The water rises and I have seen people drive by with only their drivers heads and shoulders above water. In that case the air intake was snorkeled so the truck pretty much drove across the bottom with the driver wearing nothing but a swimsuit and an SEG.

Do watch out for currents. The side of most vehicles offers a lot of area for water to press against. Don't underestimate how much force flowing water can produce. A cubic yard of water weighs roughly a ton. A bit of flotation taking pressure off the wheels, a bit of moss on the rocks and a small but steady current can conspire to produce a sleigh ride you can't stop. Sometimes your lucky if you just lose the vehicle and escape with your life.