The Germans thought so as well. I bet there were more than a few Wehrmacht soldiers who breathed a heavy sigh of relief when the invasion was postponed, only to later find they'd be hading off to North Africa or Russia. At least they would have been in an environment they would have felt more comfortable on - land!
Particularly the Navy (KM). Further entries in the diary along with the essays written immediately post war by many of the senior commanders make it quite clear that the Luftwaffe's failure in the Battle of Britain made a perfect excuse to abandon an invasion plan which they regarded as impossible and had no stomach for. They did, quite literally, blame the Luftwaffe. The Luftwaffe, or surviving officers bought in to the myth of the Battle of Britain for their own reasons. Galland used to talk about the Battle
for Britain, which never happened, though in later years he let it drop.
There has been much written in the thread about the ability of the Luftwaffe to sink RN vessels, but this is not supported by the facts.
Only three of twelve capital ships sunk between September 1939 and November 1941 had succumbed to air attack alone and these were Italian battleships sunk at anchorage. Smaller ships were more vulnerable. Twenty eight RN destroyers and five of its cruisers had been sunk this way.
The sort of bomb needed to sink larger vessels was investigated by the RAF after attempts to bomb German battleships in September 1939. The so called "Job 74 Trials" were carried out and a report made by the Ordnance Board. Essentially GP bombs exploded outside the vessel and were relatively ineffective. A delay fuse to allow them to "bounce around", maybe entering a hatch or falling over the side to create a near miss which was damaging was one solution which sounds ridiculous today. SAP bombs with a short delay were really required. Unfortunately the Luftwaffe did not have many such bombs in late 1940. The Sprengbombe Dickwandig (SD) ordnance only went up to 500Kg in late 1940. The Panzerdurchslags Cylindrisch (PC) ordnance was used against Illustrious in January 1941, but results for the 1,000Kg bomb were disappointing and the 1,400Kg version was developed later.
How were these large bombs to be delivered? By far the most accurate ship bomber was the Ju 87. It needed a cloud base at around 10,000ft for a typical attack profile, fine in the Mediterranean but not a daily occurrence in the English Channel in September. The Ju 87 B equipped the units operating from France. The B-2 could supposedly could carry a 1000kg bomb, though not according to the loading plan I have. This wasn't really big enough for sinking really large ships with any certainty, given the limited chance of scoring significant hits. Compare it with what the RAF dropped on Tirpitz!
We can forget the torpedo bombers, the He 111s of KG 26 were the only ones trained to do this, and their record even later, was woeful. It wasn't helped by the unreliability of the German torpedoes. The one thing that the RNs anti aircraft defences could hit was something coming in low (below 40 degree elevation) straight and slow.
As for hitting small ships the debate from the RN side centred on whether the smaller vessels should manoeuvre to dodge the bombs or maintain a study course. Rapid manoeuvre threw off the High Angle Control system for the ship's anti aircraft control system. The "Air Defence Instructions, 1939" did not permit fast evading action against dive bombers, despite admitting the limitations of the current anti aircraft defence systems. Luckily RN Captains are allowed considerable latitude and almost all small ships, and cruisers, did "dodge" the bombs. Rear Admiral L H K Hamilton wrote to Admiral Forbes from the cruiser Aurora in May 1940, having been subjected to continuous high level and dive bombing for thirty six hours.
"From my experience, I think that provided one has sea room and independence of manoeuvre in a ship of this size, one is most unlikely to be hit."
An enquiry established that HMS Ghurka was sunk by aircraft in the Norwegian campaign because she had "detached from screen to improve arc of fire", losing the protection of mutual covering fire from other ships. Also her Captain, a gunnery officer who believed in the efficacy of the AA system had deliberately chosen to "keep a steady gun platform without weaving."
There is an idea promulgated by some that even the best anti ship aircraft, the Ju 87, could simply swoop in and the ships were doomed. This is patent nonsense, not supported by the facts.
Alec Dennis was serving in the destroyer HMS Griffin of Crete. He remembers the Ju 87s attacking Force B.
The JU 87s came down in "groups of three, one after another........it was a classic attack, technically interesting, physically terrifying and, actually, ineffective."
All the bombs missed as Griffin weaved at top speed whilst the accompanying cruisers (Gloucester and Fiji) threw up a terrific barrage.
The light cruiser Naiad finally sunk after sustaining numerous hits over a two hour period, having dodged thirty six near misses in one ten minute period.
It took thirteen hours of attacks to do for Fiji. She had run out of AA ammunition and had resorted to firing practice ammunition (solid shot) at the incoming bombers.
HMS Kipling survived eighty three attacks in a period of a few hours. HMS Kandahar survived twenty two attacks in a four and three quarter hour period.
Aircraft did not have the ordnance to sink the large battleships and whilst they could sink smaller vessels it took a tremendous effort over prolonged periods and even then success was by no means guaranteed.
The mine fields didn't work and the KM knew it. Neither the Luftwaffe, nor the KM could not keep the RN out of the Channel and the Luftwaffe and KM knew it. If any troops got ashore they could not be reinforced or resupplied in the face of concentrating resistance from the British Army and Royal Navy and the Luftwaffe, KM, Heer and OKW all knew it.
The only reputable historian I've read (and for whom I have the utmost respect) who seriously believes that the Germans would have attempted an invasion, successful or not, is Richard Overy. He is one of those who has always promoted the Anglo-American myth of the Battle of Britain, which more or less ignores any contribution from the Royal Navy, and in this case I beg to differ.
Cheers
Steve