The original question was would the Germans fail on land or at sea. I think the Germans would probably succeed on land but fail at sea. I reckon the German invasion forces of September 1940 would stand a good chance of defeating the British and Empire forces in Britain. Fortunately for the British, there's no way to get the German forces to England in September 1940 and keep them adequately supplied. A British admiral from the Napoleonic wars put it best. "I'm not saying they can't come. I'm just saying they can't come by sea."
As an intellectual exercise, one day I want to try a gamed version of Sealion with only the land forces involved.
There are something like 25 Divisions in Britain. By September 1940, even the ones who fought at Dunkirk would have likely been fully re-equiped.
Even *if* the Germans can get an army across the Channel without losing large numbers of men and large amounts of equipment, do they have the coastal shipping to keep that Army supplied?
My thoughts are 'no'. Because if they had the shipping to keep the army supplied, they would not be considering using river barges on a roll of the dice to get across the Channel.