If France and Britain fought a serious war, the naval aspect would be focused on on raiding. France had a very large submarine fleet, much larger than the one Germany began World War Two with. France also had a more robust cruiser fleet than Germany. The Richelieu would probably be the first full sized fast battleship commissioned, notable considering the trouble the Royal Navy went through with the Bismarck. France also had metropolitan ports that gave direct access to the North Atlantic and the Mediterranean, and a global network of colonial ports. The damage to trade would be tremendous.
The British would win a naval war in the end, but it would be crippling to Britain. That kind of Mutually Assured Distruction probably promoted teamwork between Britain and France, and also explains Britains's fall from superpower status: No one expected someone as irrational and lucky as Hitler to gain power in a major country with a real navy.