I have seen some wild assumptions that it would take 24-28 hours to cross the channel. Yeah, no, you can swim over it faster than that.
Yes, one person can perhaps swim the Channel at the Dover Strait. But 130,000 people - 13 divisions plus support troops plus supplies plus ammunition plus equipment plus naval escorts - do not just walk into the water in unison and swim across. One person can just walk down a road; an army needs planning and experience in making a march or it descends into chaos as units accordion (stop/start/stop/start) or turn down the wrong roads. Moving a mob is
hard and it is
time-consuming; that's why armies invented bureaucracy. Getting a lot of men into boats is
hard and
time-consuming and that is why river-crossings are so challenging.
An amphibious operation is perhaps the most difficult of all military maneuvers - I'd include a fighting withdrawal as less difficult - and it needs to be planned, organized, rehearsed and then carried out like a piece of complex choreography. It took the Allies years -
years - of practice in England to work out how to marshall the troops, load the men and gear and stuff on the boats in precise sequence, and organize when the boats and ships sailed and in what order. And D-Day was still chaos on the beach in some places - hell, some of the troops couldn't even land on the right beach.
Vukodev, the standard Allied landing craft - the Higgins boat - was one of the most insightful pieces of genius of the War and is usually listed on any top-ten or even top-five list of war-winning things. As it turns out, the Kriegsmarine did put some resources into developing a landing craft. They built 2 - yes, 2 - before the war and they were complete failures. Germany said they were going to use tugs and river barges because that's all there was - not because it was a viable solution, but because the dictator said 'find a solution' and didn't ask, 'Will it work?'. Germany did not build landing craft for SeaLion because they didn't know how at the time. As to the docks... the Allies built artificial harbors at Normandy. They used enormous caissons of concrete, old warships and freighters and even brought their own pipeline. By contrast, Germany was hoping they could use a fishing pier and get petrol at the village garage.
Of course, if you commit
Bismarck, the
Scharnhorsts, the pocket battleships and the cruisers to SeaLion you can build a nice breakwater if you can beach them before they sink.
Look, the US Marine Corps started getting serious about amphibious warfare in the late 1920's-to-mid-1930's. They put their total resources into it, made it their reason for existence, developed a doctrine, planned the equipment and procedures and the rest... and the first amphibious landings (Guadalcanal and Torch) were catastrophic screw-ups that would have been a disaster against organized opposition. Dieppe was better in that it showed how very, very far the Allies still had to go to get good enough to even think about trying it. And even in the late-war phase at D-Day, units got confused, the beaches were a mess and the supply situation was total chaos for days and weeks after. Getting right just one thing - the combat-loading of supplies - took several tries and tens of thousands of casualties before they got it right, and the Pacific island invasions never did solve the problem of unloading the cargo ships in a timely manner.
Amphibious landings
are not simple. I get that the Soviets pulled one off and my hat is off to them, but SeaLion was going to be bigger, more complicated, more contested and much - much - more chaotic than crossing the Black Sea at Kerch. From the Wiki: "Two days later at Yenikale, over 4,400 men of the Soviet 56th Army (landed were units of 2nd and 55th Guards Rifle Divisions, and the 32nd Rifle Division), enjoyed massed artillery support from positions on the Taman Peninsula and established a firm beachhead which the German V Army Corps and Romanian 3rd Mountain Division were unable to defeat. By 11 November, the Soviets had landed 27,700 men in the Yenikale Beachhead."
Let me point out that moving 4400 men is very different from moving 130,000. Moving 28,000 men in ten days does not mean you can move 130,000 in one. If you land 4400 Germans at Folkestone the police will arrest them or the little old ladies at the tea-shop will club them into submission with their umbrellas. I mean... this is
just not reasonable.
And have we even answered
which SeaLion we're talking about? The Army wanted 13 divisions moved over a broad front, the Navy said maybe 3 on a narrow front, and neither would agree to accept the other's plan. So... the Army was refusing to go unless they could take
twice what the Allies used on better-defended beaches at Normandy! And the Navy had enough barges to move a quarter of that, if the Royal Navy and Air Force stayed away and the sea didn't get too rough. It's fantasy.
No, it's a recipe for the biggest Allied propaganda coup of WW2, a catastrophe that would rival 6th Army's surrender at Stalingrad and cost Germany the paratroops and the best of her army, the cream of her air-force and the rest of her Navy. Britain should have been
begging Hitler to try it. That sort of reverse puts an end to Barbarossa for three years and likely five, or it gets Hitler put out of a job.