The basis of any good opening is a set diplomatic workings that support a dynamic thrust to control the flow of the game and to dominate a neighbor. Standing alone as a tctical exercise all openings fall short, it is the diplomatic framework that the players must achieve that make the opening work. This is even more so in the case of the Sealion Opening names after the German invasion plans of England in World War II.
The German player must be confident that Russians are absorbed in the South and ideally having a major blowout with the Turks ovr the Black Sea. The German player also ahs to be focused on a quick take down of England and the demilitarization of the lowlands so that he can turn quickly east after the strike.
The French Player wants to see the Italians going on a long slow road to the East, hopfully a Lepanto type opening. he also wants to see the English go down like a rock and is willling to take the lead in this and the initial risks.
At this point the game for the English is over. If the English use their fleets to dislodge the Germans they can still retreat to threaten the English center and the French crush first London and then walk up the Island. If the English go after the French then the Germans walk around the Island.
Either way, England is crippled quickly, the Lowlands are demilitarized and the French-Germans have three units each to pursue their next targets.
The opening is clean, quick, and deadly with a little imaginative diplomacy on the western side.