The boat water tank usually holds potable water for on-board use, though there are other applications. NauticExpo offers a great variety of water tanks: flexible, custom made, waste water, etc.
The boat water tank furnishes water for on-board use via faucets and hoses.
Boat water tanks can be rigid or flexible. Either type will be made of materials, usually plastic, suitable for potable water storage. They generally have fill, outlet and ventilation openings, as well as a removable lid to check water level and cleanliness. They come in many shapes and sizes: 60, 80, 100 liters, etc.
Shape and capacity are the two primary criteria of choice. Other factors include interior access and a means of checking water level.