Full motion, or waveless waterbed?

Either way, it's going to depend on the natural wavelength resonance of the bed, its coupling to stabilized ship's motion (different near the centerline of the hull, than out near a beam), and the amplitude of the resultant coupled motion. But generally, your inner ear is going to be less confused by the higher frequency and smaller amplitude waves a waterbed will support, than by the slow, constant rolling of a ship. But any kind of reasonable cruise ship is gyro stabilized these days, so unless you run into major weather, or rogue waves, you're hardly going to know you're at sea.

Wait, was this some kind of subtle physics joke? Or are you trying to out ex-70's hippie waterbed salesman? Look, I sold the van and got a haircut a long time ago. We all make mistakes.

Most large modern cruise ships are heavily stablized.

On unstablized ships of yore hammocks were popular because they could be stowed and because they swung free. They make gimbled cup holders and even stoves and such for boats to take advantage of the same principle.

So, yeah, a waterbed would slosh pretty good as the center of gravity shifted, even in small amounts.

A waterbed on any small, fast, rough riding power boat or sailboat would be nigh impossible, but it has probably been done already.

My dad had a bunch of friends who were boat pilots and captains for hire and such, and one of the things about going out fishing or cruising that I simultaneously loved and hated the most was bouncing around in the foreward cabins like a superball when the ship was underway over a decent swell.

Lots of fun for a couple of hours when you were about 10 or 12, but even at that age it got old fast.

LOL. I don't know, but I'm reminded that if you're in a falling elevator and you jump up in the air just before it crashes, you'll be fine.
/kidding

I was drooling over this glorious (and cheap!) cruise. But airfares to Miami are a bitch for both my February and April breaks. *sigh*