I know you already have these covered:'
Real Creatures:Kraken, leviathan, mermaid, shark, jellyfish, toad
Sea Anemone
Plesiosaur
Nautilus (a squid relative, but awesome shell)
Rays (like a manta ray, devil ray, etc.)
Clam (other shellfish could work)
Starfish
Leech (and the basis of the afanc...a Welsh monster that was a giant leech)
Crocodile/Alligator
Crab
Lobster
Mythological
Apophis (Egyptian mythology)
Scylla (Greek myth)
Kappa (Japanese monster, grows weak when it loses the water in its head)
Boobrie (giant waterfowl, Scottish myth...eats cattle)
Charybdis (Greek myth)
Ahuizotl (Aztec monster, a monkey/dog hybrid with a hand on its tail, known for drowning people)
Bunyip (Australian Aborigine)