When Timmy is showing the king of Atlantis the problems of surface life, the king says that he eats starfish, crabs, and the occasional undersea squirrel. This may be a reference to the characters Patrick, Mr. Krabs, and Sandy from another Nick show, SpongeBob SquarePants (1999).
King Greg mentions that his people sometimes eat underwater squirrels. This is obviously a reference to Sandy Cheeks, from SpongeBob SquarePants (1999).