Wow, I love that central room with the secret door that goes nowhere.
That's just cruel, and unusual.
PCs: "We take 20; search the room."
DM: "Ok, you find against the far wall."
PCs: "Excellent, we open it, what do we see?"
DM: "A unadorned stone wall."
PCs: "No way! I disbelieve."
DM: "A stone wall."
PCs: "Wow, this is impressive. I attack it with my axe."
DM: "Your axe breaks."
PCs: "Damnit!"
PCs: "Break out the pick-axe boys, this is gonna take a while."
DM: *sigh*
There was a "treasure map" that TSR put out several years ago on a cliffside, and the interesting thing about it was that it made use of the tides. So, the particular dungeon was only accessible in low-tide. So, you could give them access to the place, and then raise the tide, sealing the exit. That room with the pool would be an excellent place to put an Aboleth. Your players wil hate you forever for that one.
The ToH also has aquatic mummies, which are quite nasty. And there's a variety of eel in that book which also incurs mummy rot on successful bites. Basically, you can F-up your party REALLY quickly with some of those nasties.
Maybe, the entranceway is accessible, and then while they're in it, the room with the pool in it floods as the tide rises (the pool is actually connected via an underground tunnel to the ocean).
Basically, as they explore, have the dungeon fill up (give them plenty of clues, the rooms reak of saltwater, the stone is really badly corroded and wet, etc.). If it fills up quickly enough, you can trap them and drown them all if you really want to. The room in the bottom left has a shaft which leads up. Too bad it's the home of a Beholder....