The Collaborative International Dictionary of English v.0.48:

Water tender \Water tender\ (Nav.)
   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.
   [Webster 1913 Suppl.]