The typical 900Mhz Baby monitor will have no problems working accross a couple of cabins.
On our most recent cruise, we took a baby monitor if for no other reason than to let DS sleep in the cabin while we listen out for him from the verandah.
While on the cruise, I put the distance of the monitor to a test. Using a $25-35 Fisher-Price monitor, I left the base in cabin 5630 (last plexi-glass verandah before the navigator;s verandah. On a whim, I wanted to see if it would be possible to watch a movie in the Beunivista Theater and still monitor DS from the cabin. From the aft (right) side of the theater, I could monitor the cabin; however, I was at the edge of the monitor's range and therefore there was too much static to effectively use it for that purpose.
But for a distance of a couple of cabins, a baby monitor will work just fine.
I must say, I was suprized at some of the things radio signals could do on the ship. I found it interesting that using 1 watt GRMS radios, DW from cabin 5630 could NOT contact me while I was in the hall near the attrium. However, if I was in Diversions on deck 3 on the Starboard side of the ship, we could talk over the radios just fine.