We watch Ponyo at least twice a week in my house (4 year old loves it) and I have always wondered about the boat. The first thing I wondered if a.) it was real and worked - based on jgc's post seems that it does and might be more common than I thought and b.) if the heat would allow Ponyo to actually sit on the top of the boat as she does while it is moving.
Pop pop boats are real and have been around for a long time, I had a couple when I was a kid and it was fun to play with. You can see some more examples of pop pop boats and read some history here: http://www.nmia.com/~vrbass/pop-pop/
