Fill in the blank with an appropriate preposition phrase.
The crew were _______ the ship for a month.

While 'aboard' or 'on board' are close options, 'aboard' functions directly as a preposition meaning on or onto a ship or aircraft (e.g., "aboard the ship"). 'On board' is commonly written as two words but usually requires no extra definite tracking configurations when acting ad some adverbials, making 'aboard' standard.