Someone who carries explosives (as from the magazine to the guns on board a warship) WordNet 3.0 & Open English WordNet

A powder boy or powder monkey manned naval artillery guns as a member of a warship's crew, primarily during the Age of Sail. Wikipedia

The persons employed to carry gun powder from the ship's magazine to the gun deck during a battle; in the 18th century Royal British and U. S. Navies, this task (also carrying water) during battles became a permanent nickname for the ship's cabin boys and apprentice seamen. Wiktionary