Look at the size of the D Day fleet (supported by total air superiority) needed to invade Normandy. Hitler did not have the fleet, he did not have the air power
He couldn't launch an invasion of England only 30 miles away and these morons think he could have pulled off an invasion of the US from 3000 miles away
Assuming Dointz got this 300 Uboats by early 1941 and starved England, I think the most likely scenario was the Germans held out against the Soviets long enough to get an A-Bomb, and then there was a status quo with the US.
Had the Germans been able to win the Battle of Britian, I don't think invasion was impossible. The Home Fleet was vulnerable to air attack ... even more so that our Pacific Fleet battleships. But, even though the Brits left their tanks and artillary at Dunkirk and France, the Germans loses in crossing the channel would have not made victory a sure thing. I'm not sure Hitler would even have chanced it. He wanted a negotiated armistace. That of course was just another of his political/economic miscalculations ... or delusions.