Again, terebella = dungeon dressing.
So, shall we bother giving it a few stats. If the SRD can include the Shrieker, we can do out own submarine dungeon dressing.
The only significant ability mentioned is its entangling tentacles, but I'd give it a watered-down version of a Sabella's poisonous flesh as well, to give it
some protection from predators.
Large Vermin
Hit Dice: 3d8+6 (hp 19)
Speed: 0 ft.
Armor Class: 12 (-1 size, +1 Dex, +2 natural), touch 10, flat-footed 11
Base Attack/Grapple: +2/+8
Space/Reach: 10 ft./5 ft. (40 ft. tentacles?)
Special Attacks: Entangling tentacles, poisonous flesh
Special Qualities: Accelerated healing, tremorsense 60 ft., vermin traits
Saves: Fort +5, Ref+4, Will +1
Abilities: Str 14, Dex 13, Con 15, Int —, Wis 11, Cha 2
I'm thinking its entangling tentacles would trap everything within 40 feet that fail a Reflex save, probably Strength-based so DC13.
It could also make a 40' range touch attack and then grapple, but I think it wouldn't get Improved Grab since it basically waits for food to drift to it on the currents, so its tentacles are not built for whipping around prey like our monstrous version of the Sabella.
If we do give it poisonous flesh, I'd suggest something like 1d3 minutes of sickened for primary effect, 1d3 Strength damage for secondary effect. Assuming any of the PCs actually bite it.