Depends on several factors. First and foremost is what is stated in your divorce settlement. Don't recommend your departure out of country with children without his written acknowledgement if not also his approval. If you're going to Mexico, a parent letter is mandatory. Travel agent, cruise line, and/or consulates office could advise you about other destinations.
Congrats on selecting a cruise. I'm taking my boys this coming New Year's aboard the Elation.......which also required a letter from their Mom. Unforged of course. Oooh, don't forget the birth certificates or passports.