It's a sump for water draining out of the ice box, and perhaps from other places. On the plans for Whisper there was a hand pump immediately to starboard of the companionway stairs to empty it. I assume it was to keep anything that spilled in the icebox (milk?) from smelling up the bilge.
I took the cover off, and don't use it for anything. I drilled a small hole at the bottom to allow it to drain into the bilge, so there is nothing in it to freeze in the winter.
Rick
