What about damaging the RAF to such an extent it could not protect the RN, thereby inhibiting an ability to stop a seaborne invasion (coupled with airborne landings)?
There was no realistic scenario by which the Luftwaffe could have protected an invasion fleet from the RN in the fall of 1940.
There were no Luftwaffe anti-shipping squadrons in service on the Atlantic coast. The Stukas couldn't even stop merchant shipping along the channel coast.
The Sealowe invasion plan relied upon river and canal barges that were not at all suited to sailing on the ocean. Most of the canal barges, a significant portion of the total, did not even have motors, as they were typically towed by horse/mule teams. These were to be towed or pushed across the channel by tugboats. But there were nothing like enough tugboats, so they were to be towed in sea-trains of 3 to 5 barges per tug. The Germans never came up with a practical method of managing the sea trains once they got to the beach area. Anything beyond calm flat seas was a significant flooding risk. Anything more than 2 or 3 kt of current or wind would make it very hard to manage the barges (hello? Have you ever been on the English Channel?). There were not enough barges to land the total intended force in one lift, so the same barges were needed not only for the initial landings, but to land the multiple waves of reinforcements, and to land a continuous flow of supplies.
That means that the barges would be crossing the channel every day for 2 or 3 weeks if any useful results were to be achieved. Any and every barge lost meant fewer reinforcements, and fewer supplies. Here there was a devil to pay, because attrition in the barges over time would mean more troops ashore (more barges early on), which would require more supplies to be landed, from a fleet of barges that gets smaller as time goes on.
Somewhere late in the planning they came up with the interesting idea of blowing the bows off of the barges that carried vehicles (the all critical Panzers, but also armored cars and trucks) to allow the vehicles to drive directly off onto the beach. This idea was put into the plan before anyone explained how a barge with no bow was expected to continue to serve in the fleet, or even how it was expected to clear the beach to allow others to land in its place.
Oh, and it took 2 days to cross the channel. Which means it was an overnight trip.
Even if there had been a fully-scaled fully trained Luftwaffe sea-strike capability, it would not have been effective at night. Two or three destroyers getting in among the tug-towed river barges of the invasion fleet were all it would have taken to shut down Sealowe. They wouldn't even have had to shoot their guns. Just running past (and even over?) the barges at speed would have swamped many, disorganized and disbursed others, and put the kabosh on whole thing.
A realistic planning expectation would be 20 or 30 destroyers and cruisers sailing to intercept the barges. So now the Luftwaffe would have the job of stopping ALL of those ships from getting among the barges (I don't think the Luftwaffe didn't manage to sink that many warships in any full year of the war, even after they had fully developed and operational maritime strike squadrons), without damaging or panicking the barges, while they were at the same time providing tactical air support to the forces that did manage to get ashore (who would not initially have artillery support ashore), as well as fighting off any RAF attacks from airbases that were outside of the range of the German attacks (but not out of range of the invasion area). And they would have to do this every hour of every day and night, because you need to defend every barge at all times against an opponent who only needs to sink each barge once.
If the Germans had somehow managed to get ashore (surprise, British bungling, whatever scenario you want) the most likely result would have been a limited beach head filled with an immobile Wehrmacht with few vehicles, starved for ammunition, distracted by the need to scrounge for food, fuel and medications. After a few days ashore the fighting would just taper off to nothing. It would have been Germany's first major loss of a multi-division force. And it would have been a very complete loss, because there wasn't going to be any retreat across the beaches, since it would have been the loss of shipping and/or control of the seas that had put them in that predicament in the first place.
Of course this is all based on a correlation of forces. Odd or unforeseen events do happen in military (or any) operations. But planning on the unforeseen is beyond "wishful thinking" and into the realm of "magical thinking" (ie: we'll win because luck will be on our side).
The balance was just too greatly weighted to the Britain's advantage on the water. The Sealowe plan was non-sense.
-Mark
(aka: Mk 1)