I wouldn't make it Disguise, for a similar reason as Shade. Make it a flat DC like camo.
Traumatizing Charge it is.
Following Shade's vote:
Damsel Lure (Ex): A giant damselfish can flutter its body and dorsal fin so it appears to be a drowning woman, either human or elf, to viewers on or above the water's surface. Viewers can attempt a DC 20 Spot check to notice this "lady" is an imitation; these checks succeed automatically once the viewer is within 10 ft (though usually the damselfish attacks by that point).
Challenge Rating: x
Advancement: 3-6 HD (Large)?
A giant damselfish averages 7-1/2 feet in length and weighs about x pounds.