It's hard to say what you'll be able to walk to. The only port I've been to at which the Western itinerary calls is Key West, and there is not a beach within walking distance there.
I never buy the ship's excursions, because I like to just hang out at the beach for a while, and then wander around the town in the afternoons. At most every cruise ship pier, there will be a line of cabs waiting to take passengers wherever they want to go. You can do a bit of research on the ports and figure out which beach you'd like to visit, and then just grab a cab when you get off the ship. This is usually much less expensive than booking an excursion throught the ship. In many ports, the cab driver will arrange a time to pick you up from the beach, but if not, there are a lot of cabs around for you to get back to the ship.