I used to make a habit of finding pianos in public places to play. (I even got in the habit of carrying a screwdriver, because a lot of common problems on grand piano actions can be fixed with it. Not recommended for the beginner, though!) I’ve only been asked to refrain a couple times. My general rule is, if it’s not locked and it’s in a public space, if there’s someone nearby to ask (e.g., hotel clerk), then ask. (Best “no” response I got to that was “Sure, if you can show me your union card. Insurance rules.” Which is probably BS but still a good answer.) If there’s nobody to ask, I would play. I still do that at hotels where I’m staying. Playing a variety of pianos is good for one’s technique in terms of versatility.
I’ve been only one cruise. A grand piano in a lounge was unlocked, so I played it. One data point!
I bet you wouldn’t leave your Martin unaccompanied in a lounge. I doubt anyone working as a musician on a cruise would have a personal acoustic piano aboard. However, if there’s a stage and a band is set up, one must keep one’s fingers off all the gear there! About the only excuse for touching a pro’s gear is if it’s in imminent danger (it’s falling, something is falling on it, etc.)