Few posts ago I stated that by saying successful Sealion I mean landing in Britain and hold few southern cities. So it won't be conquering England but some sort of stalemate where both sides can not defeated other. For some time of course. And I know that after landing on British soil Royal Navy will be mad and will try to sunk all convoys in Channel. But this would be excellent practice for German naval bombers. Of course many of them will be destroyed by AA guns on British ships but then Germans will be forced to improve their tactics, weapons, engines and train better pilots. I think that jet engines could be introduced earlier by Germans in order to defeat Allied forces. I think that RN after some defeats will learn how to defeat wolfpack and after that Germans wil build better submarines (as OTL). Also I think that if Germans allocate all efforts to Britain during 1940-1942 instead of waging war against Yugoslavia and USSR (or even signing secret alliance with them) they could be able to enter Great Britain. With big losses but still.
The problem for Sealion is that the logistics are already on the point of failure
without the Royal Navy.
Okay a probably not complete list of reasons why Sealion would have been a failure:
1) There are simply not enough transport vessels, even if there were the ports of Northern France are filled to capacity. This means the first wave assault troops will be waiting days (the Kriegsmarine estimate was 10 days) for their organic supply units to be delivered.
2) The nature of the vessels being used means that they will be crossing at night, against an enemy with radar. The British can find them, their own bombers cannot.
3) Back to the supply vessels, many though not all are river barges for inland use one good storm would literally capsize Operation Sealion.
4) The British Army has numerous prepared defences, is well dug in in the coastal ports and has numerous transport links to bring up reserves against any landing areas. The German bombers will struggle to stop the British Army concentrating and then it has ten days to destroy the assault forces...which note are without supply for this period.
5) The Royal Navy is huge, really really big and nor just does it only have battleships but lots of cruisers and destroyers too, on top of this it also has numerous small craft that can easily attack tugs and ferries even if they cannot take on warships. Oh and submarines, the Channel is not a great space for subs but the British ones can cruise around on the surface at night shooting up tugs and barges.
6) The English Channel is in range of 11 Group oddly enough, if the Luftwaffe let off suppressing 11 Group then its fighters can cover the Royal Navy against bombers by daylight. Worth noting Fighter Command do not have to even shoot down any bombers just so long as they jog enough yet not even all pilots that a sufficient force of the RN is able to operate in the Channel....not that the Luftwaffe had a great record against mobile warships or even at this stage against stationary ones.
7) Due to the chronic shortage of landing vessels the British do not at any point need to get them all. It will take at least 6 trips without casualties to get all the German land forces proposed for Sea Lion across the Channel and then more trips to resupply those forces in combat. If the number of operational vessels drops below the resupply threshold (a threshold which btw they never met to begin with) the invasion fails.
As for German improvements all of these were the work of years not months and that was an issue when Germany needed to ready to invade the USSR by 1942 or contemplate the utter failure not merely of Hitler's master plan but likely the War against the British as well...though they should be a good few years from formal defeat their lack of economic endurance meant that the Reich would not be winning...shades of WW1 all over again.