Voracious creatures that sink ships and devour their crews

Sea Monsters are a general class of creatures that may be encountered on an ocean voyage. Some are serpentine, others constructed more like squids or octopi, and still others like truly huge crabs or lobster. The GM should choose a type, then select features that seem reasonable. For instance, a gigantic crab would certainly have the best armor class (2), and might have 9 hit dice and do 4d4 points of damage with each of its two claw attacks. Or, it might have two different-sized claws doing 3d4 and 4d4 points of damage respectively. Whatever form they are found in, sea monsters are always voracious monsters who seek to sink ships and then eat whatever creatures they find thereon.