I think Sea Lion *could* have succeded but not on the timetable that was being planned by the German High Command.

The Battle of Britain was the key point and this should have continued concentrating on military targets, early warning and airdefense, infrastructure (roads, railways, oil depots, powerplants).

The Luftwaffe's bomber arm shouldn't have begun doing "deep" strikes until some sort of air superiorty was won. If the fighters had been concentrated on at least denying the channel and immediate ground of southern England to the RAF they could then have used their advantages of higher fighting position, superior numbers, and more experienced pilots to draw out and kill the RAFs fighter arm while minimizing the huge downside of the Bf 109s extremely short range. The bombers would in the mean time concentrate on eliminating any RN presence in the English Channel, southern North Sea, and the southern approaches to the Irish Sea. This should have continued for several months. Just having done this would at least allow the Kriegsmarine to operate closer to English waters and would have allowed at the minimum commando raids on coastal targets, especially radar.

Using this time the Germans should have designed and built or at least copied true landing craft and tried to build them in quantity. Barges made for canals are simply not suffiecient for the English channel and too easily destroyed to be considered for an Invasion force.

If the above objectives were at least partilly achieved and sustained then at least the uboot arm would have not needed to fear air attacks from the RAF while operating near France and Southern England.

Then of course if true airsuperiority over southern England was achieved an invasion force would have been able to cross with out interferrance of the RN and with the state the British Army was in at the time once the intial resistance was crushed (which with air superiority would have been achieved in my opinion) the Wehrmacht would steadily push north once their armor was landed and suffiecent supplies for inland operations were stockpiled. If the Wehrmacht had gotten a good foothold on English soil with a secure naval supplyline (made secure from the air) then England would probably have fallen fairly easily.

The biggest failures of the real Battle of Britain/Sea Lion was that Georing and Hitler had "victory disease" from the Battle of France and thought that air superiorty would be quickly achieved quickly and that the RAF would be beaten within 3 months. They let bombers operate beyond the range of their fighters and underestimated the effiency of British radar (which without the real BoB might not have been won). The later switch from military targets to terror bombings shows the lack of patience and overconfidence that the German High Command had. Then of course once Barbarrosa was launched any invasion of Britain in the near future was impossible and with the addition of the Americans in 42 they had to switch to defense.

With a protracted timetable with realistic goals (not commiting to deep operations too early) and expectations the Germans could have won. I'm confident that with the British needing supplies from overseas and the loss of fighters and more importantly pilots that Germany could have one.