1) Surface-dwelling people (like the PC's players) definitely tend to think in 2 dimensions for most encounters. Most sea dwellers will think in 3 dimensions, even the "dumb predators." Have hostiles attack from oddball angles, above and below.
2) RE: Octopi and other Cephalopods
As I posted in another thread (link below):
I was talking to a guy who worked at an aquarium who was discussing some footage of a giant (6') octopus that was eating sharks in their main tank...he also showed some footage of an octopus climbing from a quarrantine tank to another one about 20' away (across dry land) into another quarrantine tank to devour its inhabitants...then crawling back. He also noted that one of the aquarium's young interns had been attacked and nearly dragged into a giant octopus' tank- it grabbed her with 5 tentacles while anchoring itself with its other 3 and started pulling. It took several other people to extricate her from its grasp.
3) Even real sea life can be very dangerous.
Incredibly powerful poison is more commonplace- Sea Wasp & Box Jellies, Blue Ring Octopi, Cone Snails, and many others have stings/bites that can kill humans, and everyone knows that some critters like nudibranch, sea snails and puffer fish have toxins within them that make them unpalatable or even fatal to consume.
Then there are critters like mantis/pistol shrimp, which despite being small enough to hold in your hand, can deliver serious injuries. A pistol shrimp can strike hard enough to break tempered glass or
human bone- similar in force to a small-caliber bullet (hence the name).
4) While you're at it, check out the rules for Anthropomorphic Animals in Savage Species- some of the critters listed are aquatic, like humanoid Octopi or Sharks. Imagine a small tribe of Anthro Great White Sharks, or an enclave of Anthro Octopi who could mistaken for Illithids, of course.
5) Also, check out these threads for all kinds of critters and other ideas for aquatic adventures & campaigns:
http://www.enworld.org/showthread.php?t=202233
http://www.enworld.org/showthread.php?t=200844
http://www.enworld.org/showthread.php?t=194546
http://www.enworld.org/showthread.php?t=185243
http://www.enworld.org/showthread.php?t=181377&page=1
http://www.enworld.org/showthread.php?t=179818
http://www.enworld.org/showthread.php?t=179508
http://www.enworld.org/showthread.php?t=179402