Operation: Sea Lion was the German plan to invade Britain in World War II. Early in AH.Com history, newbs constantly brought up Sea Lion in Axis victory scenarios, leading to the point where merely mentioning it without stating it's utter impossibility will send an AHer nto a rabid fit.
As such, Sealion became memetic, and is now used as a comparison for any type 1 to 3 level of plausibility.
The major difference between Sealions and Alien Space Bats is in possibility. ASBs are explicitly impossible; that will affect the reception of the ATL, but all the work involved is from the ASBs onward.
Sealions aren't explicitly impossible. With the right Point of Divergence, Germany could conceivably have launched a sea invasion of the British Isles. The problem is that it would have to be much farther back, and would have had lots of knock-on butterflies in the meantime. By the time the invasion is possible, the war being fought would NOT be WWII, and the Germany sending its fleet across the English Channel would be very, very far divergent from anything we'd recognize.
Calling an ATL a Sealion is a serious accusation; it means that to get where the author wants to go, large parts of the backstory need to be rewritten from the top.