From Alternative History

Sealion stands for the German plans to invade Britain durign WWII.

Several times discussed in soc.history.what-if untill it became part of the FAQ, chances that Operation Sealion would have succeeded are between slim and none.

For Sealion to be successful, Germany would have needed a good plan, control of the Channel, Air Control, and resources to ship men and ammunition through the Channel.

Just to make a beachhead.

Then the Germans would need to fight the British on British soil.

Note that while Germany was successful with her Blitzkrieg against Czechoslovakia, Poland, Netherlands, Belgium and France, all those territories lie on the Great North European Plains... good ground for the panzers... which is not the case of the English Channel.