Well, I might as well add in my $0.02, while I'm at it.
First off, Operation Sea Lion won't work. Why? Because it absolutely can't.
Let's put two points first.
Germany does not have the aerial capability to put down the RAF for good.
Germany does not have the naval capability to even put a dent in the Royal Navy's overwhelming superiority.
Now, moving on to alternatives.
The Isle of Wight? Well, much like the infamous Friesian Islands thread, this is an objective that is within range of mainland artillery, also serving the purpose of telegraphing your next targets, making it a horrendous target that will most likely be reclaimed within two weeks.
Gibraltar? Well, for starters, let's assume that Hitler suddenly developed an addiction to drinking lead paint. To consider attacking Spain, you'd have to divert manpower and resources from Sea Lion, most importantly, fuel, which will undoubtedly exacerbate the fuel problems of the Germans. But the Heer will also be in a terrible logistic situation, since Spain has been thoroughly wrecked and its rail network is in shambles. But here's another issue. If you attack Gibraltar after France, then you risk missing the window for Sea Lion, because if Sea Lion wanted to have even the slightest sliver of a chance, it would have to launch before the conditions in the Channel made it impossible. Oh, and you also end up with more than likely massive famine in Spain, that will force the Germans to divert food there. In addition to losing the very few resources that the then-neutral Spain could get to Germany, since Spain acted as a conduit of goods to Germany, making it far more useful as a neutral than as an enemy that would doubtlessly inflict some casualties that Germany will rue by the time Zhukov comes a-knockin', or even an ally, since Spanish operations inevitably would have had to rely on German supplies, most importantly, fuel. Which wouldn't help, seeing as Germany was going through fuel fast, draining their own reserves and the captured French fuel reserves.
What's stopping the Germans from launching a second Sea Lion in 1941 if they fail in 1940? The very same thing that stopped them the first time. The Royal Navy. Only this time, the Germans' condition will be much worse, due to having suffered the utter annhilation of their makeshift landing craft. Can't they make more, you ask? Sure, but at the cost of negatively impacting the civilian economy and taking badly needed resources from a U-Boot or some other weapon that's far more vital than a ship that's doomed to end up in the bottom of the English Channel, anyway. Because, procuring materials for landing ships will inevitably take resources from an ongoing production.
It's not comparable to Normandy, because the Allies set this up for years, having naval and aerial supremacy, with Germany having neither. It's not comparable to Barbarossa, because the Soviets are a continental power that are on the same landmass as the Germans, as opposed to the British Isles. It's not comparable to the Pacific theatre, because it is a whole different animal. It's not even comparable to Crete, because Crete was the Germans against a less than well-equipped force(understatement, I know), and even then it was a close-run affair.
Now, to the main event.
The river barges are flat-out death traps in the English Channel. A near miss could end up in their sinking. The mere wake of a destroyer could probably make them capsize. The Kriegsmarine can't provide naval cover for the invasion force because Britannia rules the waves. Submarines are a no-go from the get-go, because they need room to maneuver and the shallow waters of the Channel make it a very bad idea. So the whole support for the operation falls to the Luftwaffe. But if you want it to fight the RAF, bomb London, support the invasion and attack the Royal Navy, this will end up in these tasks being done badly, since it can't do all of them effectively, so aircraft losses will start to pile on, leaving Germany in a worse strategic position post-Sea Lion.
The river barges are slow, unmoving targets that will take...what? Roughly 48 hours to get to their landing zones, depending on the tides. While the RAF will certainly not be the killing blow for them, it can certainly attack the invasion force throughout these 2 or so days, which will certainly not be good for the men stuck on those death traps called barges, because this kind of attack is sure to diminish their fighting capability.
The Luftwaffe against the RAF? Difficult enough for the Germans as is.
The Kriegsmarine versus the Royal Navy? Laughable.
Operation Sea Lion? Unfeasible.
There, the dead equine has been struck once more.