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

"Aboard" means on or into a ship, aircraft, train, or other vehicle. While "on board" functions similarly as a phrase, "aboard" is standard and concise here, and "were aboard the ship" is a conventional structure. Both "on board" and "aboard" can represent presence on a vessel, but "aboard" functions directly as a preposition in this context before "the ship".