This is an opening in the outside of the ship below the water line that allows sea water to enter the ship. Sea water is needed for many reasons on board ship and the sea chest allows this to happen.

Sea water is distilled into fresh water. It is used for cooling. It is used for the fire main system. It is used for the sea water compensating system in the fuel tanks. All of this is possibly because of the sea chest.

Since it is a direct connection to the sea the Navy uses what they call, double valve protection. Two valves are installed back to back just inside the hull from the sea chest. This is to prevent a flood. One draw back with a sea chest is they all junk to be sucked into the ships water system. However there is a set of strainers just after the two valves. This needs to be cleaned from time to time and this brings about an Urban Ledged that runs around the Navy.

In locations where there are sea snakes they are sucked into the system and caught in the strainer. When Fireman Schmuckatelli goes to clean the strainer he is bitten by the snake a dies on the spot. Since I never made it to any of these areas I cannot state if this claim is true or not.

One prank we pulled on all sorts of new guys was to send them looking for the keys to the sea chest so we could open it. Oddly enough they never found them.