I wanted to use a different humanoid creature for this, but didn't like the skum in the Monster Manual I. So I modified him. My skum is the same, except for these additions:
Skills:
Hide: 20*
Climb: 20**
* Mutated skum have a chameleon-like ability that allows them to blend-in with their surroundings, both in and out of the water. They enjoy using this ability in ambushing tactics, although they are not as talented in moving quietly.
** The claws of the mutated skum can be drawn outwards, which makes them curled into the form of hooks. With these, skum can climb walls and ceilings that are made of normal stone or weaker material. They may climb thus at a third their normal movement rate.
Special Abilities:
Acid Splash: Once every three rounds, mutated skum may spit a glob of acid that will inflict 1d3 points of damage to every target within ten feet of its impact.
CR: 3
Level Adjustment: +4