Ive done it just when germany was taking over the french ports
I actually never encountered a ships except one time near a city (cant remember the name) when 3 destroyers were sunk by coastal batteries but i cannot confirm were the batteries friendly firing at the allied DDs or the other way around.
The best thing to do (as far as i am concerned) for crossing the chanel is to stay as close as possible to the german(french) coast.