One of the saddest days in WWII.

 France, a great power, at least before June of 1940, owned a powerful navy.  Not quite as big as the Royal Navy, but far bigger than anything Hitler had at his disposal.   With the French fleet under his control, Operation Sea Lion might have worked.  The Germans possessed a very small fleet, and the British had been making it smaller.  They sank the Graf Spee and at Narvik HMS Warspite had steamed up a fiord, cornered and sunk most of the Germany destroyer fleet.  In the summer of 1940, the Royal Navy had better than 100 destroyers, backed up by thirty odd cruisers and half a dozen battleships.

The Germans had practically nothing.  The Royal Navy would have had no trouble sinking anything the Germans had in mid Channel.

On the other hand, if Hitler had laid his hands on the French fleet, he would have had a chance.  The bulk of the French fleet was at Oran in North Africa.  The British send a large fleet to Oran and told the French Admiral Gensoul in command that he had three choices, sail his fleet to British ports, sail to the French West Indies (Martinique) or sail to the United States.  Otherwise be sunk right there at Oran.   The French tried to stall for time.  The British were in no mood to put up with that, they opened fire and sank the entire French fleet right then and there.  You would have thought that after 1000 years of dealing with the British, the French would have understood what they could and could not get away with.  Apparently not.  Needless to say, this ruined Anglo French relations for the rest of WWII, but it did keep the French fleet out of German hands.