I have seen some people doing things on their own and not make it back in time (once in Cozumel due to flooding). They were left behind and had to catch a flight to the next port. They also had the added cost and hassle of finding a room for the night in Cozumel. They didn't make it back to the ship until we were about to leave the next port so they missed out on that too. I realize this is the extreme case but it does happen. Just something to consider.