Questions about terrain...

[zeiss]

So, how deep of a stream can a vehicle cross without going under?

It's a matter of movement cost and chance of immobilization.
–1 ford; lower move cost/ immobilization chance,
-2 stream; higher move cost/ immobilization chance..

So, does terrain as well as movement itself affect accuracy on target?

No. Accuracy depends on how many movement points the unit spent prior to firing. The fewer the better.. difficult terrain drains more points of course..