Put the point of departure in 1920 have france england and belgium decide they need more farmland. They begin filling in the channel. By 1940 they've completed the project. When the panzers reach Dunkirk they keep chasing the Brits west till they reach the coast of Wales.
Austria produces Red Bull earlier than OTL, some crazy nazi bartender creates a prototype Jägerbomb, which quickly becomes the SS elite's drink of choice. It makes them more vigourous, and by the grace of St. Hubert, they swim across the channel... Churchill is hooked on Jägerbombs, and declares Germany as a perennial ally.
It could have worked if they had simplied planned to invade England prior to 1940. They had no plan in place and there were no preperations being made until the fall of france was guarenteed. But diverting resources to a naval invasion force would mean that another area would be lacking. I think if they took the time to develop a really good land based naval attack aircraft to atleast hold the Royal Navy off. If they planned and had time for the preperations it could have worked. It would mean abondoning the invasion of the USSR, and I wouldn't be suprised by the time all the men needed to supress the Brittish Isle were on the ground there that the Soviets would have launched there secret attack which they were planning on doing at some point. So it would be possible but would probablly have no bearing on the war. The only reason the Allies invaded main land europe was to secure western europe from the Soviets, by that time most higher ups knew it was a matter of time before the Soviets would win. I think the reason the Germans scrapped it was because what is to gain by capturing GB? THe loss of men would be great and at the time the UK possed little threat to the Germans anymore. I think they realized they had bigger fish to fry, so it is ASB to think that Sealowe would have worked before the fall of the USSR because there is so little to gain with the fall of Brittish Isle compared with the conquest of USSR.