Some others that are different than described above:
1. turtle -- "timeout" sign -- right hand flat & vertical, left hand placed on top to form "T" shape
2. turtle -- think "shaka" but put one hand on top of the other and move your thumb and pinkie up and down (the four flippers) -- sometimes done with just thumbs
3. eel -- one hand, forming mouth, opening and closing (like the symbol used when someone keeps talking on and on)
4. crab -- like eel, but with both hands at the same time... may also refer to lobster.
5. shrimp -- like crab, but only using thumbs and forefinger fingers
6. stonefish (scorpionfish, turkeyfish, etc.) -- left hand flat and horizontal (palm up), right hand make fist and hit left hand... incorporates the "danger" signal into it, but better describes the danger
7. octopus -- use hand in vertical motion drawing all five digits together, then apart, moving up in the water
8. squid -- like octopus, but moving side-to-side
9. "little" creature -- hold thumb and forefinger close (1" apart) and then point
10. trumpetfish/coronetfish -- hold thumb and forefingers of both hands like (9) close together, then draw them apart, indicating an elongated fish
11. whale (or other marine mammal) -- simulate blowhole by putting hand, palm up on top of your head, then rapidly opening and closing it
12. "hear the whales" -- cup your ear (hear) then do (11).
some of these are illustrated at
http://www.oxforddivecentre.com/signals/signals.html#Marine