It is a longish yellow inflatable banana-shaped rubber raft. A group of people climb and/or hang on while a panga motorboat pulls you around the bay. You bounce with the waves and sometimes you fall off because of the jarring. The ride is usually between 15 minutes and a half-hour depending upon how much time you negotiate.
The closest thing I can think of (stateside) is a boogie board being pulled by a motorboat.
I hope this helps explain it. If you do a Google search that includes banana boat and Puerto Vallarta, you can probably find a picture or two.