The Imap Umassoursua is a mythical creature from Inuktitut mythology. It was described to be a very large flounder. It is the size of an island and incredibly flat. It might have colossal wing-like fins that it uses to swim with. It has an enormous cavern-like mouth that it uses to vacuum up ships to eat. It was reported off the coast of Greenland.
It could be legends based off of the Kraken. It is found in the same general area and the Kraken is notorious for being confused as an island. It could also be based off of either the Aspidochelone or the Jasconius, which are both fish that are often mistaken for islands.