Depends on your bubble ( I mostly use blobby surfaces, with sampler and info, blinn material, look at the tut on mike's site and make the colour of the water a bit lighter, they'll look as bubbles ). If you create an invisible nurbs plane the place where you want them to pop, just make them collide and die on impact, or, for a nice effect, make them die on collision and emit 10 new, 10 times as small particles that die after a very short while.. I hope you know what I mean