The story
From the fact that the species is a diver, the largest or greatest of those regularly found off the British Isles, which has a very northerly range (no further south than the British Isles).
Etymology from Wiktionary, CC BY-SA 4.0.
great northern diver
Today it means: A large member of the diver family, Gavia immer, commonly breeding in North America, where it is known as the as common loon. Full definition →