Here's a first run at my room...what do you think?
Compared to the swirling horror of the whirlpool, this tranquil cavern seems the perfect opposite- here, the waters flow through a gigantic geode of deep purple amethyst crystals.
The tiny faces of the millions of crystalline structures surrounding them reflect the party's light sources like tiny stars, and the effect is mirrored in the dark and gently flowing waters upon which they travel gives the effect that the party is traversing the night sky in their tiny boat.
About the middle of the cavern, the enchantment is broken as nasty black spears pierce the surface of the water from below...
The attackers are typical Skum, armed with longspears. There are 9 of them, grouped in 3's. They are using the reach of the longspears to attack the boat while remaining below the water. The water's darkness & distortion of the surface makes fighting difficult. Furthermore, the Skum's longspears are primitive, with flaked stone heads. (Use the primitive weapon breaking rules of the DMG.)
(To reflect the difficulty of this kind of fighting, treat the water's surface as providing soft cover- IOW, a -4 to attack. However, with each successful hit, reduce the penalties by 1, and eliminate the penalty if a hit results in a crit threat. Since the Skum have some experience with this, they start off with only -2.)
The Skum are trying to damage the party, to be sure, but they are mostly trying to herd the party towards their unseen 10th member. To that end, they are attacking on 3 sides, leaving the 4th side (nearest the opening to the next cave) open.
Submerged at the entrance of the next cave is the one they call "The Beast" Nabbar. He is a Large Skum (6HD), Str 21, Dex 15, Con 15, Int 10, Wis 10, Cha 6
Unlike his smaller bretheren, he is mildly psionic- the end result of selective breeding including psionic stock by his Aboleth Master. Instead of Alertness, he has Improved Natural Attack: Bite (now 3d6), Wild Talent, and Psionic Fist (expend psionic focus to have Unarmed Attack or Natural Weapon deal +2d6 damage).
As an alternative to Psionics, give Nabbar the usual Alertness, plus Improved Initiative and INA, but give him the Half-Fiend template. He is the end result of the breeding program including a number of would-be sorcerers.