Inspired by someone over at Dragonsfoot, I made a list of the monsters in Supplement II. I have not yet compared the lists. I have also not listed the monsters from the Temple of the Frog, although some are replicated here:

Mermen
Giant Crabs
Giant Octopi
Giant Squid
Giant Crocodile
Giant Toad
Giant Frog
Giant Leech
Giant Beaver
Giant Otter
Giant Wasps
Giant Beetle: Giant Stag Beetle
Giant Beetle: Rhionceros
Giant Beetle: Bombardier
Giant Beetle: Fire
Giant Beetle: Boring
Lizard: Fire Lizard
Lizard: Minotaur
Dinosaur: Elasmosaurus
Dinosaur: Mosasaurus
Dinosaur: Plesiosaurus
Giant Shark
Whale
Giant Eel
Lamprey
Sea Horse
Portugese Man of War (Manatae)
Dolphins
Aquatic Elves
Pungi Ray
Manta Ray
Water Spiders (Giant sea Spider)
Weed Eels
Sahuagin
Floating Eyes (Fish)
Ixitxachith
Locathah
Morkoth (or Morlock)
Masher
Poisonous Coral
Strangle Weed
Lycanthrope,
Nymph (as Dryad)
Mottled Worm
Gnonme
Kobold
Leech
Ochre Jelly
Green Slime
Evil High Priest (Underwater!)
Roper
Gelatinous Cube
Sea Hag (As Dryad)
Kopoacinth (Underwater Gargoyle)
Koalinth (Underwater Hobgoblin)
Lacecdon (Ghoul)

-Havard