For most vehicles, air intake is in the engine compartment. Belts, fans, alterntors, etc. will splash a lot of water up so that it could get into the air intake. If it does not flood the engine, it will wet and seal the air filter… all not good for an internal combustion engine. I imagine it only takes a few splashes of water in the cylinders to make it difficult for the fuel to burn. Plus, as others mentioned, you are going to short out electrical for gasoline engines, so no sparking. (Alternatiors are open air and I imagine don’t alternate well immersed. Dirty water might short out between the exposed battery posts, especially if it’s salt water?
Another issue I fould when driving around one day in deep puddles with a manual transmission Honda Civic - the clutch was not well sealed, and in one very deep puddle I suddenly felt the clutch slipping like crazy - water between the plates. Fortunately I made it out and the water squeezed out in a few seconds… Brakes will have the same problem - often in very wet weather the first second or so the brakes may not work.
Basically, some vehicles are designed for extreme water situations, most are not.