Originally Posted by
Rheinstorff
It's worth noting too that the Kriegsmarine was a force to contend with, as it demonstrated to the RN regularly from 14 October 1939 until at least mid-1943, and would almost certainly have contributed to the neutralisation or destruction of the RN with its combination of submarines, fast attack craft and world-class major surface units (Bismark, et al).
The latter would have proved unlikely to show up for Sea Lion as she only started sea trials on 15 Sep 1940 with Tirpitz a few months behind. The other issue the RN faced with the Kriegsmarine was that they could be anywhere so concentrating enough assets in one place to neutralise even a U-boat was an enormous task, this wouldn't be the case if the invasion had gone ahead as you'd know exactly where they were going to be.
However what was most likely to cause the failure of the German invasion plans was lack of experience and equipment, even with the right equipment Dieppe was a disaster for the Allies, imagine doing it without proper landing craft.