At sea, when the boat may lean over or roll in the waves, the opening may end up below the waterline; if the seacock is open when that occurs, seawater could flow the wrong way--into the boat.
The opening may end up below the waterline. This is at sea. The boat may lean over. The boat may roll in the waves. Seawater could flow the wrong way--into the boat. This is at sea. The seacock is open. That occurs.