Yes, they "try" to just make a foot dunk, but sometimes it doesn't stop at that point. It's entirely possible that you can get more than just your feet wet. Plus, just being in the little speedboat you're going to get splashed. You won't stay dry.
Another point - that little speedboat is really rough riding. You're hanging on for dear life, and bouncing up and down on the waves. I know I came away with bruises. And one of the women on our boat banged her head when we had to shift seats while the boat was traveling.