Can Somebody Tell Me What This Is.

[ben2go]

We may be have 2 separate elements going on here.
My understanding of a seacock is as defined by Wikipedia, "A seacock is a valve on the hull of a boat or a ship, permitting water to flow into the vessel, such as for cooling an engine or for a salt water faucet; or out of the boat, such as for a sink drain or a toilet. Seacocks are often a Kingston valve".
Therefore, I'd rather have my seacocks located below the waterline.
We are a powerboat so heal is usually not an issue with us.
I do have seacocks on my sinks and bilge pumps as well and those are above the waterline.
Therefore, a sea chest, which is what started this line is used as a below the waterline source of water, thru seacocks, to provide a single point of entry for that water.
A boat with a sea chest will not have the sinks or bilge pumps seacock going to the sea chest.
Hopefully this will clarify my original answer about a sea chest.

Some boats do have a sea chest or standpipe to take water out of the boat from sinks and other sources or waste water. The one that shouldn't be in a sea chest, at least in my mind, is the overboard for the head.