Enochlophobia (Crowds): Specifically refers to the fear of crowds. When crowded areas are the trigger, it often manifests as feeling restricted by the presence of people, rather than the walls themselves.
Well, I’m not crazy about crowds; but the thing that scares me about cruise ships is simply not being able to get away when something bad happens - like an outbreak of disease; or suddenly needing serious medical care.
There doesn’t seem much to attract me on an ocean cruise that I couldn’t find on land.
(Some of the little US river cruises look interesting, though.)