It depends on the tour. Most ship excursions have a defined start and end date, meaning if you've booked the beach getaway, then you'll likely be meeting up somewhere at a specific time to get your transportation there.
There are some excursions billed 'at your leisure' which means you can choose when to go, but transportation for such things may not be included.
If you've booked it, you will pay whether you go or not, so make sure you actually want it. Take some time to research what you really want to do. As to missing the ship, I wouldn't let that make your decision. Just ensure you leave yourself plenty of lead and return time before you are due to set sail. Also make sure you are aware of ship's time vs. local time differences.
RCI Sovereign of the Seas 2005
Caribbean Princess 2005
Carnival Paradise 2004