Actually, even if Germany had achieved local naval superiority with the assistance of the French Fleet - in itself unlikely, as while the French Fleet was the fourth largest in the world, the Royal Navy was
the lrgest, and could take them in a straight-up fight - the Germans would still have been thrown back into the sea. Ad-hoc as British anti-invasion plans were - they'd never expected the Germans to perform so well, so had not prepared for the eventuality at all - they were still more than enough to prevent a successful German invasion. They were, to put it bluntly, also pretty damn brutal, involving such wonderful (and illegal) things as mustard gas.
Germany wasn't really capable of amphibious invasions. They got lucky in both Norway and Crete, the only amphibious invasions they launched against opposition. The British withdrew from the Channel Islands rather than try to defend them when they were closer to France than Britain, giving Germany aerial superiority.
In Crete the Germans were being beaten decisively until some New Zealand troops, under the mistaken impression that they were facing a significantly greater number of paratroopers than they actually were, withdrew from a strategic position, giving the Germans control of the local airfield. The Germans then rushed in reinforcements and supplies. They also had local aerial superiority over Crete, which, like the Channel Islands, was closer to German Continental airfields than British ones.
In Norway the Germans were assisted somewhat by the fact that Quisling provided some political and military support - though far, far less than he'd promised - and the fact that the Royal Navy simply wasn't in the area at the time. Even then the Germans suffered heavy losses; I believe they lost the use about a third of their surface fleet during the invasion, though most of it was salvageable. They also benefited from Churchill's poor strategy to counter them; he concentrated on taking Narvik, which he did, when he should have denied the Germans the men and materiel to maintain their grip on the city, forcing their withdrawal. The Germans also never fully captured Norway; its mountainous terrain was perfect for guerrilla activities, which continued until the end of the war.
What these two invasions proved was that the expenditure necessary for the Germans to launch a successful invasion over the sea was far in excess of what was needed to fight them off. And Britain was a much tougher nut to crack than Norway or Crete.
http://en.wikipedia.org/wiki/British_anti-invasion_preparations_of_World_War_II