As someone who once worked tangentially in search and rescue, please do not even consider this. The ocean is a serious thing, doubly so at night. Unless you are renting a boat large enough to come with its own staff, please do not just head over the horizon simply to see the stars. And fyi, the stars at sea move as the boat you stand on moves. They are brighter, but also more blurry.
Cruise ships dont have dark decks or other places to view the ocean directly at night. It would be like standing on your porch with the exterior lights of ypur house left on.
Avoiding light pollution is not really about seeing stars through light pollution. Thats for astronomers with telescopes. For human eyes it is more about being dark enough thay your iris can relax and let in more light. Try a dark forest, even a city park, surrounded by trees but able to see up. You will see more stars even if inside an urban area.
Also an area that is dark enough for ‘far enough’ that your eyes will naturally adjust to pick up these faint light sources. While some adjustment happens even in a few minutes, the difference between that and after several hours of darkness is mind blowing.
Most folks in a city likely have never been able to experience being able to walk by true starlight on a moonless night, and seeing clearly. It would be nearly impossible to get the right conditions even with a lot of effort.
A city park may be okay ish, but you’re unlikely to ever get the level of sight you’d get walking on a deserted playa in the desert. Not enough time with true darkness, and too much other light pollution.
Just drive an hour away from your nearest city, to the "rural" parts of your state. That's all you need to see a gorgeous night sky. You will see a beautiful sky even just a mile or so outside of a small town of 10k people.
Did you already have sea experience, or did you just rent a boat and gave it a try?