Good idea but as has been stated before, you would have to think of a type of business plan as to what you would provide for consumption. Bacon butties, Chinese, Indian, pizza and the list goes on. You would, I presume, be at the mercy of the Environmental Health rules about cleanliness, 2 sinks one for washing hands and one for washing pots etc.. Would you need a traders licence for operation on the canals or rivers?
It will need a great deal of thinking, planning and internal alteration to your boat. Fridges and freezers to keep raw meat separate from cooked stuff, cookers and cool cabinets to keep food stable before selling.
Best of luck anyway and if it does come to fruition, let us know where you are operating from as I'm sure that we all will patronise your establishment !!