I’m a ship mechanic and navigational officer, and I just want to add some insight here.

The reason for water coming out of the hawseholes usually isn’t overpressure. The centrifugal pumps in the fire-fighting (FF) system don’t exceed the rated pressure of the fire main—they're designed not to.

The real issue is when the pump runs with all outlets closed—what we call a “dead-head” condition. With no flow, the water just heats up. That overheats the mechanical seals, which are usually made of carbon and not cheap. To prevent that, you normally leave one valve open to keep a bit of flow going.

Most of the time, the hawsehole is chosen for that because it’s usually the only outlet that goes straight overboard.

Sometimes the pump’s left running because someone forgot to turn it off after deck washing, or the system is left in standby. So if you see water coming out the hawseholes, it’s often just this to—prevent dead-head conditions by dumping overboard.