I took a dash of the SRD and added a pinch of the ACK vampire to get these suggested stats.
% In Lair: 80%
Dungeon Enc: Scourge (1d4) / Nest (1d6)
Wilderness Enc: Cavern (1) / Underground Lake (1d4)
Movement: 180’ (60’)
Armor Class: 6
Hit Dice: 9*** (Att. Throw +2)
Treasure Type: M
An aboleth has the Swim Proficiency.
Enslave (Charm): Three times per day, an aboleth can attempt to enslave any one living creature within 30 feet. The target must succeed on a Spell Savings throw or be affected as though by a Charm person spell. An enslaved creature obeys the aboleth’s telepathic commands until freed by remove curse, and can attempt a new Spell save every 24 hours to break free. The control is also broken if the aboleth dies or travels more than 1 mile from its slave. Remove Curse also breaks this effect.
Spell Casting: Confusion, Phantasmal Force, Feeblemind, Hold Person, Hallucinatory Terrain
Slime: A blow from an aboleth’s tentacle can cause a terrible affliction. A creature hit by a tentacle must succeed on a Poison/Death save or begin to transform over the next 1d4+1 minutes, the skin gradually becoming a clear, slimy membrane. An afflicted creature must remain moistened with cool, fresh water or take 1d6 points of damage every 10 minutes. Remove disease reverses the affliction.
Mucus Cloud: An aboleth underwater surrounds itself with a viscous cloud of mucus roughly 1 foot thick. Any creature coming into contact with and inhaling this substance must succeed on a Poison/Death save or lose the ability to breathe air for the next 3 hours. An affected creature “drowns” on dry land in 10 rounds if removed from the water. Renewed contact with the mucus cloud and failing another Poison/Death save continues the effect for another 3 hours.