You're soliciting advice to haunt Cedious with again?
The Stone Collosus thing is an excellent idea, because it will screw up all their water-breathing magics. I also recommend Mummy-templated Minotaurs of Legend as eternal guardians of their masterworks ala-Egpyptian tombs, and Drowned from the MM2 (or indeed the Grasp of the Emerald Claw module if you have it).
If you want a guardian of ruins, you can always bring in one of my patented
Astraloths. It's not like the Giants, at the height of their glory, weren't capable of binding such powerful outsiders, after all.
Since it's not capable of healing quickly, and the Astraloth in question is likely to be too powerful to defeat in a single battle, it would allow them to engage in a guerrilla-style tactics at high levels without being too cheesy. It works especially well underwater, where it has even more of a manuverability advantage over that of an average adventuring party.
Aboleths would be a great way to introduce the Sahuagin as their mind-controling master. If you want a fiendish theme, make Aboleths the spawn of Khyber instead of monsters from Xoriat; Khyber is perfectly capable of producing things like Cloakers and Medusas in addition to the Rakshasas and their Rajah rulers.
From the mist, insert air elementals or invisible stalkers that are a result of Xen'drik's cataclysm and have full concealment within the mists.
You need any more ideas?
EDIT: Oh, I thought you might like this picture.