I agree that a cruise ship is not the ideal way to "sail" the Carribean.
But if you think of it as a big hotel that magically appears at different locations every day, feeds you, and entertains the kids, it can be a good family vacation. We've been on several, starting when our daughters were 3+4. The kids loved the supervised play areas, and we liked having the option of some time alone. Sure, you only get a few hours at each port, but you can scope it out for future trips. Hint: Bring a bottle of your liquor of choice. It will reduce the bar bills significantly.
If you can, plan time to explore the fortifications of Old San Juan. They're not far from the ship.