They could have taken the UK and Ireland easily with what they had. That would leave the US without a toehold anywhere in Europe. Then Hitler would only have had to sit tight and hold parades and rallies.
However, that is not the sort of thing that megalomaniacs tend to do....
Letting the Allies escape from Dunkirk was Hitler’s first big strategic mistake. Their Navy would have had trouble getting an invasion force across the Channel (basically, it would had to have been D-Day in reverse). Instead of building up their capability in that area, Hitler listened to Goering who claimed that England could be bombed into submission. But his biggest mistake was invading Russia.