Hadn't thought of the shampoo route, and I think you'd need a bunch of ethanol. How about diatomaceous earth? I've never had good luck with it, but perhaps there it would work.
Or go classic...a giant Fresnel lens! Or parabolic mirror from shore! Lasers! Greek fire!
Well, the idea of a surfactant is to get the water to invade the pore spaces filled with air that keep them afloat. Sink the nasty buggerers.
Of course, fire would expand those air bubbles, possibly breaking up the raft, but causing the air to expand and be expelled, in part. Then when they cooled off, those which were not immolated might find the air bubbles, reduced in volume (as they cooled), provided less buoyant force and that they, too sank. However, if they could take a bit of air with them, they might survive in spite of the immersion.
Diatomaceous earth would irritate them if it got in exoskeletal joints, if it is full of pyrethrum might do some good, or could be a carrier lattice for other toxins.
I think the trick is to get their bubbles to burst or be displaced by water due to capillary action.
Kids with powerful lasers or focused sunlight might be a winner (as long as they aren't zapping each other).
