a small floating dock, and a target holder, you could use cardboard as a target backer and put shoot and see stickers on it, and a small anker or cinder block to keep it from floating away. No worries about littering, as with the balloons. How about a RC boat with a target holder on it, so you don't shoot the boat? I'm working on a RC truck, I just found a gyro for rotating left and right, and to raise and lower the target holder. I'm using a power meter bracket for the target holder it's not too heavy and holds the cardboard target backer . A work in progress to be sure. Hope this stimulates some ideas for you.
TODD