As he ran up the stairs, Joseph saw a bunch of divers heading in the opposite direction. In his adrenaline rush, because some of the divers didn't ignore him, he used gravity slingshot to simply push the entire group back up the stairs.
While that would alert any divers on deck, the tossed pile of divers should act as a temporary shield.

As such, when he reached the top of the stairs, he let go of his suitcases, using gravity slingshot to swing them at two of the divers he could see, while using his flight ability to cancel his momentum, and pull himself back into cover behind the stairs.