We lived on Calypso II on the canals and rivers of France, Holland, Germany and Belgium for 8 years after we retired.
I didn't brew any beer because with wine at about 80p a litre I was very seldom sober enough to even consider it! :whistle:
IF you are on a canal (as opposed to being at sea) then there should be no problem whatsoever with brewing beer and cider on a suitably sized boat. (i.e. one that doesn't roll too much and one that has decent open-air areas - preferably covered.)
We generally used filtered canal/river water for most "washing" duties (toilet, pots, pans, clothes etc) and only used fresh water from our tanks for rinsing. I can see no reason why you couldn't do the same for brewing equipment.
We also used gas onboard and even had a charcoal BBQ set up on the afterdeck so again using gas for heating and boiling water for brewing should be no problem.
Finally, we discovered that the bilges (which were totally dry) were an ideal place to store wine and beer because, regardless of the temperatures outside and in the cabin, they were always around the temperature of the canal or river itself.
Best of luck with the venture. Here's a photograph of Calypso II. :thumb: