I have no experience in this area of the profession, though I know a number of actors who have, and they all seem to enjoy it, and make decent money from it (as well as get to see interesting parts of the world by doing it!), so there may be many a good reason to take the job on.
What I will say, in response to your fear that screen casters will dismiss you because of such work on the CV - I think the only thing screen casters ever judge an actor on is their screen credits, by and large (perhaps your headshot to some extent, in order to ascertain your look). In other words, if you are genuinely interested in pursuing a screen career, you need to be looking into developing a showreel to showcase yourself, and getting as much experience as you can of working on screen projects.
But that's not to dissuade you from going on the cruise ship. It's simply to say, they are different areas of the industry. Your best option might very well be to work on the ship this year, earn good money doing it, and then use some of that money to tide you over when you come back home, and look to start getting a showreel together (which might mean having to deal, initially, with some student or low budget setups, who are always chronically strapped for cash with which to pay you!).
In contrast, from a stage perspective, I think citing your cruise ship experience will probably be quite welcome at the lower end of the industry, on the fringe etc. - it will show that you have ensemble experience, are a team player, a versatile performer, and can cope with audience expectations - all of which are very, very important stage skills. Yes, perhaps it isn't the sort of thing to be over promoting when you are applying for a job at the RSC, but I'm sure you can cross that bridge when you come to it.