In the United States navy, a first-class petty officer in charge in a fireroom. He "tends" water to the boilers, sees that fires are properly cleaned and stoked, etc. There is also a rating of chief water tender, who is a chief petty officer.
A water carrying vehicle capable of resupplying engine companies with water when other water sources are not available. These units range from 500 gallons to over 5,000 gallons.
A water tender is a specialized firefighting apparatus designed for the sole purpose of transporting water from a water source to a fire scene. Water tenders are capable of drafting water from a stream, lake or hydrant.