The squids are drawn with beaks but the octopus doesn't have one. In reality, all cephalopods have beaks. This was perhaps a conscious decision by the animators to distinguish the octopus from the squids.
Nemo is a reference to Captain Nemo, the figure from Jules Verne's classic novel, Twenty Thousand Leagues Under the Sea.
Mr. Starfish is a reference to Mr. Starbuck, the first mate of the Pequod in Herman Melville's classic nautical novel, Moby-Dick.
Sealand is based off real life theme park SeaWorld.
Captain Finn mentions his submarine only has 20,000 leagues on its odometer. That is a reference to Jules Verne's novel, Twenty Thousand Leagues Under the Sea.