I'm resurrecting this because I had the same problem with my first pour. My guess, prior to coming in to read this thread, was that the plastic was too hot. If Pitbull's pour was like mine (and the end result looks very similar), it poured fine, mold filled fine, then the bubbles appeared. It just looked to me like it was 'boiling'. I warmed the POP molds prior to pouring.
I did seal the mold first with straight Elmer's, then after reading this thread, I cut in the water and flooded. I made the mold about a week ago and only sealed the top side.