A German seizure of the French navy would have consequences. The main reason for French agreement to the armistice was to ensure the continued existence of the French Empire in some format after the war, and the navy was essential to that. They had thought the war lost and that Britain would soon make peace ... being lenient on the French navy was important for France to accept the armistice as it was incredibly harsh in other respects.
So if the Germans get their hands of the French fleet, the French colonies will almost assuredly defect to the Free French.
So while it is bad to have lost the French Fleet, the Allies are actually strengthened in Africa and the Middle East.
As others have pointed out, it will take a long time before Germans could adequately crew the ships, and the are divided into Atlantic and Mediterranean Fleets.