We were just on Zuiderdam and the walkie talkies were spotty at best. My wife had a conference on deck 3 and I had lots of trouble communicating with her on deck 7 (in our room). I think that the bulkheads cause too much interference.
Baby monitors will be useless unless you are in the room next store. However, I have never had a problem on any ship I have been on using a 3 watt or better radio. Can go through every deck from front to back of the ship.
I guess my question is why would you leave your baby in the room and go out somewhere on the ship while they were sleeping. Maybe thats not what you are doing, but being that fires, etc., can happen on a ship, I sure wouldnt leave a little one.
Like many things, it depends on the quality and ppower of your unit(s). Our son & wife have successfully used the baby monitor and all of us communicate easily with our 5-mile 3-watt VHFs. However, some countries you may visit have laws which outlaw their use without a license. (we just carry our FCC operator's license, but have never been questioned nor heard of anyone who has. This last info from having cruised our own boat in foreign waters; they check smaller vessels more than cruise ships)