We did this when my DD was just turned 10. As I recall, she weighed about 60 pounds at the time. I sat in the seat behind her (the very last seat). Each person has to be in their own seat. She held on fine, but was a little freaked out. There was a girl about her age on the same boat, and once DD saw that the other girl was having fun, she settled down and had fun too.
Couple of tips though. Wear goggles. That salt spray gets into your eyes and burns (that was the biggest thing DD was upset about). And wear shorts (or something that covers your inner thighs. There's usually sand on the banana thing, and bouncing up and down with your thigh rubbing the sandy rubber skin of the boat, it will cause a rash!
It was lots of fun though!