User Manual BaoFeng BF-888S Two Way Radio with Built in LED Flashlight (Pack of 4)

It depends. These are, like most radios, line-of-sight. If you are on one side of the mountain, and the other radio is on the other side, it's not going to work. If you are all on the same side with no obstructions like hills, they'll work great.

Yes, the earpiece does have a mic. I have never used those that came with the radios, however, so I cannot attest to their efficiency. I already owned earpiece mics - that look like secret service/CIA type earpieces - and I continue to use those. I am sure they work fine though, everything else about the radio is acceptable.

Mine came with a US 120v plug only.

My team uses these radios for communication for several different exercises; communication between and in buildings is clear and effective though usually distances less than 200 ft. While traveling by vehicle on the interstate system communication between vehicles is also clear with distances usually no more than one mile. I cannot say with any certainty whether or not these would be effective on a cruise ship between floors; I wouldn't see any problem across the ship, however. The ship's communication system could also be a factor, albeit a small one. However, for the money, these two-way radios have proven themselves to be a bargain and effective. Might I suggest you simply ask a representative of the cruise line about the effectiveness between floors? I'm confident two-way radios have been used before and they may be able to provide you with a definite answer.

Not sure if it works on Mac. I set up mine with the software I found for Windows. The software that came in the box wasn't useful.

Yes, you can hook up a headset. Some of these even come with them. Look for a radio with the earpiece included, and they show you the plug.