I think it would hurt if you didn't let go in time and slammed into the boat. That being said I would love to see the video of someone doing it. It would be very funny. Maybe one could even submit it to america's funniest home videos.
I built a zipline from the back of our house to a tree that is in the lake. It is a ton of fun to ride. The only thing I would be worried about is how tight you could get the zipline. We have it tied off to two trees and it was tough to get it really tight. If it isn't tight you won't go very fast.
you might no be able to do it anchored, but I have done it by just leaving the boat in gear and having the roap tied to a bridge so its creating constanst pressure..... the rope will still sag getting near the boat, and the person hits the water well befroe getting close to the boat.
Ziplining towards a boat in gear just doesn't sound like a good idea to me. You could anchor it, but before you connect the zipline, make sure both the zipline and anchor line are tense, therefore the boat won't move very as much.
tried that...it doesn't take much boat movement at all to get too much slack in the zip line. We couldn't get it to work. Instead we found areas on the shoreline where there was a small cove with a high tie off point across the water from an anchor point near the water. In one case we found a large log near the water and used the houseboat stakes to "block" the log. That held it in place well enough for the zip. Because the line was so high, we also had to tie a length of line to the zip handle so we could fling it back up the zip line to the top start point. Hours of entertainment for the kids
You should try to eliminate the boat from the equation. Tie the zipline strait to the anchor. Take up the slack on the shore and you should be good. But tie a bumper to the line to keep people from boating over your line.