We always pre book this. My children love the snorkeling in a safe environment. You have 2 floats to chose from. A flat one or a circle. The flat one is better. I have yet to do the bike ride. My DH, DD and DS do tis while I relax on the beach.
This is not something you have to book a head of time. Although we have always prebooked. You can wait to book it while onboard. You might have to wait in line to book on the ship. If for some reason you do not dock at CC they will credit your onboard account.