What's that tap?

[Rod]

Whilst recently cleaning out the 3 water tanks on my MKIII TH and trying to sort out the maze of hoses etc in the keel compartment behind the keel tank I was wondering about a tap that seemed to simply go into the side of the keel casing. No idea what it is for?? It was still in good repair and did turn but I didn't venture to open it properly as it looked like it was connected to the outside of the keel for salt water. Is this likely/possible?

[Dolphin]

Rod, If its on the starboard side it is probably the sea cock for the cooling water for the diesel. Usually a smaller one, about 1/2inch.