Using a VHF transceiver on a narrow boat has some peculiar problems.
First is that the narrow boat will affect the radio transmission because its steel and will act as a ground plane in the lengthwise direction. This means that signals from the side or to the side will be weaker than fore and aft. This can happen with handhelds as well as fixed radios.
Secondly, the antenna fixed to the roof of the boat will graze the underside of bridges and some tunnels, so will need to be lowered. You can of course use a short spiral wound antenna but these are not as efficient as a proper part wave antenna (half or quarter).
Thirdly, hand helds will have a tendancy to go missing. Either at a lock or off the side into the water so will require attachment to something with string. This takes the portability away from the use. A fixed radio will work better but then becomes a target of theft and also will require the use of a longer mic lead and an external speaker.
Hope this helps.
I forgot to add that on a fixed installation, the antenna must not be next to the steerer or anyone else as it has a heating effect on flesh when transmitting. In all cases ensure that the antenna is away from the eyes. of anyone.