Hostage/prisoners shackled to the anchor chain of a ship, or trapped in the chain locker. Drop the anchor and a person
will go through an eight inch port along with the six inch chain. Not neatly, but they will go through.
Giant hermit crab using a small ship as its 'shell'.
Large octopus that grabs a PC. Doesn't attack, doesn't do any damage, just decides to take a ride.... (I know folks this has happened to - octopi also like crawling inside of the goggles....

In the real world being a 'pods joyride was actually one of the high points of their trip.

) Hard to get off, huge grapple bonus, and a -2 circumstance penalty to a lot of things 'cause you got a cephalopod on your back/arm/face/in your pants....
Aquatic rot grubs....
Aquatic carrion crawlers....
Our old friend, the mimic....
Oozes are almost a given.
Giant blenfish (hagfish, slime eel).... Incidentally, their slime can be use as a substitute for egg whites.
Some variant of the fat innkeeper worm (urechis caupo), and its dependents. The worm 'hunts' by blowing a big bubble of snot, then deflating it, trapping much smaller critters in the snot. It is a more effecient hunter than it is an eater, as a result the worm is attended by a large number of opportunistic feeders.
The Auld Grump
*EDIT* A troupe of bad tempered, vicious monkeys living in the rigging of several ships. Flinging rocks, shells, rotting fish, and less mentionable refuse at the PCs. Monkeys are horrible critters, sometimes.