Operation Sealion (Operation Seelöwe), Nazi Germany's plan to invade the United Kingdom in 1940, is one of the most derided PoDs ever. Some reasons for this are the horrible planning for it (the Germans planned to cross the Channel in Rhine river barges), the very low chance of success, and how often AH n00bs have called it plausible.
Anyway, here's the Glossary of Operation Sealion Discussion Threads - a (growing) collection of PoD discussions about this particular military plan and whether it makes any sense or whether it could have succeded at least partially.
For another discussion on the topic, go here.
BTW: The operation originally was supposed to be called “Löwe” (lion), but renamed because, y'know, they'd have to cross the sea for it. Apparently nobody noticed that a sealion isn't really a scary animal.
The phrase “successful Sealion” is generally used when describing an extremely implausible TL or PoD. In most cases Sealion is a taboo subject. However, there are some exceptions.
Sealions (along with Zeppelins and some other symbols) have become the unofficial mascot of the Board. As of July 2006, a small, stuffed plush sealion, known as 'Sealion', embarked on a round-the-world journey. This trip will terminate at IronYuppie's residence where Sealion will be presented to the Overlord (praise-be-to-his-Dark-Name). This happened in 2011.
Sealions appear throughout AH.com The Series as a recurring gag in many episodes, such as the episode “SEALIONS ON AN AIRSHIP”. One of the sequels, the AH.com Enterprise show, also features two main types of shuttlecraft in the Twin Fleet, dubbed the “Sealion class” and the “Rhine Barge class”.