A watertight door system for a marine vessel comprises a door and a door frame having a threshold. A circumferential seal is around an opening of the door frame closed by the door. The seal is formed between seal components of the door and seal components of the door frame. The frame is adapted for connection to the bulkhead and comprises two sides, a top member and a floor trough. Means are provided to press the door against the frame when the door is in a closed position. The trough has a floor plate substantially flush with a top edge of the trough, which floor plate is adapted to form a floor panel of the floor of the vessel, with said trough received in an aperture of the floor. The trough has a lid to close the trough, at least when the door is in an open position, so that the floor is rendered substantially level through the door aperture. The seal may extend along the lid (in the case of a pivoting door) or in the trough, in the case of a sliding door.