If he wants all the creature comforts of home and is never going to be away from the dock......??? WTF
It is doubtfull if he will be able to sequester the needed number of shore power outlets and not get the other slip owners ire. It is also doubtful if he will have the room for a 20 gal tank.
It is likely that an on demand unit can be made to work provide you can vent it correctly and meet all the safety requirments for gas appliances in boats.
Is he going to be needing a 14 CF reefer also?
how about a lazyboy and 56" plasma? (you did say ALL the creature comforts)
blender, toaster, garbage disposal, trash compactor (how big is this boat?) flush toilet, microwave, coffee pot, walk in closets, .......
The number of "creature comforts" that you give up when limiting yourself to an effiecncy appartment (aka boat < 70') is pretty large.
My first impression is he is not going to find that the reality of living on a boat meets his dream expitations but one can always hope.