One cannot know what the Unmentionable Sea Mammal is, for it is the bane of Alternative History incarnate.
However, the base for it, and what we really mean by this term is the never attempted ((for VERY good reason)) Operation Sealion, a planned German Invasion of Great Britain during 1940. The reason that we call it the Unmentionable Sea Mammal is because of the sheer number of AH NOOBZ that always use this nearly impossible invasion as the reason the the Nazis win WWII, which IMHO is utterly impossible.
Thanks for the information.