Attached Link: Larger Animated Boil

You can get the surface disturbance through the booleaning.  I've attached a still shot from the larger animated gif (528k, at the link).  Notice the half popped bubbles at the surface.  It doesn't look like much in the still, but the animated version has an ok boiling/bubbling effect, which could be adapted for whatever you want.

Edit - I really like AS's thing, too :)

Come visit us at RenderGods.

Ignore the shooty dog thing.