Storage - Water is usually kept in a storage tank on board.
Connection - On all but the tiniest of boats this tank will be connected to a tap at the sink, where it is pumped out by hand or by electric pump. On a hired narrow boat you will find electric pumps, and you can expect to have both hot and cold water.
Capacity and Replenishment - The boat's water tank will be of a suitable capacity for the number of people aboard, but it will need replenishing from the water points which occur fairly frequently on most waterways. Large cruisers and narrow boats will have a water filler point on deck, and will carry a hose to connect with the water point. There is also a filler cap for fuel; they should be both clearly marked and well separated, but take care not to mix them up! On small boats where the tank may only be accessible from inboard, you will probably find it easier to fill it with a portable one- or two-gallon container - a funnel helps with this.
Water Points - Water points are shown in the navigation guides, and it is good practice to make a daily check on the water situation and the whereabouts of the next tap.