The problem is Magic, Hitler needed to get boots on the ground, and that meant Seelöwe had to work and for that he needed not only Aerial supremacy but Naval supremacy too, which he never had.
Of course when you compare Kriegsmarine to Royal Navy, the first one turns out to look rather sad. Just let us take the sheer number of destroyers that were ready to engage in battle in the 1940, with England having about 40 of them out of 70 stationated near the Isles and Germany a stunning number of 10. OK, maybe 11. :) Talk about huge disproportion, innit? This situation looked rather similarly with other naval units, but that is just why Hitler decided to set everything on one bloody strong card, Luftwaffe!
At first, Hitler didn’t plan any invasion on the British Isles. He always used to say that on the decay of the British Empire everyone will gain except Germany. Hitler was bluffing and by threatening The Britain he wanted to force them to sign a pact on his conditions of course. An agreement between those two countries, where the GB would still rule the seas and III Reich would rule the land. His admiration of British nation stopped however in the moment when Britain overruled all of his proposals for that treat. Hitler really didn’t like it when something was not going according to his schedule, so he totally changed his plans. He knew that the key to conquer Britain was by the air force. If he would manage to eliminate or even strongly damage RAF Royal navy wouldn’t be as much of a problem for him and the possibility of an German invasion would be very serious. We could see it perfectly in the pacific war what a ship is worth without the backup of plains, vide the history of Bismarck, Tarent, Pearl Harbour, Yamato or Prince of Wales with Repuls. Besides, at that time most of the British battleships weren’t equipped with proper antiaircraft artillery and the armor of the deck was usually to thin( vide Hood being sank by Bismarck after only one salvo!).
In conclusion, by having powerful air force the Nazis didn’t need to have likewise powerful naval force to carry out a successful assault, and after taking under consideration that Germans were able to beach land about 300 000 soldiers and 8 500 vehicles, the situation of GB would look very grim. Personally I don’t think that it would be a light campaign for the Wermaht. History showed us that Britons can really fight, and taking under consideration that they would have a bloody good motivation to do so I am far from predicting their defeat, but the damage that they would inflict would be definitely big. Fortunately, RAF didn’t fail the expectations of it’s high command as well as the British citizens, and this long paragraph is just a fiction. :)