A canteen was a small container used by soldiers for carrying water or other liquids. Military Assault Command Operations soldiers were equipped, at least occasionally, with canteens.
In 2153, MACO F. Hawkins took a canteen that had been fixed to the belt on his uniform, opened it, and briefly gave the canteen to T'Pol to drink some water from, while they were aboard the D'Kyr-type starship Seleya. After the canteen was used by T'Pol, Hawkins returned it to his belt. (ENT: "Impulse")
While rock climbing on Earth the next year, Starfleet Captains Jonathan Archer and Erika Hernandez took a canteen with them. (ENT: "Home")
Archer and T'Pol also took a canteen with them during an expedition on Vulcan. (ENT: "The Forge", "Kir'Shara") While T'Pol was being held prisoner by Talok and a detachment of Vulcan commandos under his command, Talok offered her a drink from a canteen he was carrying, though T'Pol refused to partake from it. (ENT: "Kir'Shara")
In 2372, Caylem gave Kathryn Janeway a canteen with water after she regained consciousness in his shelter. (VOY: "Resistance")