Ok let me try to spin a positive out of @Zachary ’s point of bad for the environment:
What if instead of big ships this is a smaller sized cruise line? Something smaller that went to educational with the ecology of what you were visiting. Offers of being able to do positive things like beach clean ups, ocean clean ups.
I know that I’m likely hoping for something unlikely here but just trying to think of a way to spin this more positive.
As someone who's been on a few cruises I can tell you right now guests aren't looking to do any cleaning anywhere especially if they're dumping thousands of dollars for a cruise. They wanna enjoy themselves by partying and drinking and visiting ports in other countries where they can shop, dine, relax on the beach and visit historical sites. Some ports even offer fun outdoor activities like ziplining, parasailing, snorkeling/scuba. etc...
If it's just licensing deals, that's fine. It could make some money for the brand. I could see cross-promotions and package deals, etc.
Anything other than this, sounds way too risky.
As I said in a previous post, if they do a joint-venture with a major investment firm they could be successful. This has worked very well for the Virgin Group as they partnered with Bain Capital to help launch their Virgin Voyages cruise line. One drawback with this is if they started now it would take a while to get going as it takes two to three years to build a new cruise ship, so your suggestion of licensing deals and cross-promotions seems more feasible with an existing cruise line. The question with this is... what cruise company will be willing to work with United Parks as the major ones don't really need them as they're already making money.