I can't imagine why you would want to close it while underway -- seems to me it would only invite an overheat if (when) you forget to reopen, and you'd want to have the ability to fire up the engine quickly in an emergency.

The seacock you DO need to close while sailing is the one(s) in the head compartment; if you are healed to port you may start to flood through the sink drain or head. (DAMHIK)

You should really have it open in case you need to use the engine in an emergency such as mast breakage or Man Overboard.

However, I do occasionally shut mine on a long open water stretch. The reason is that I once had a mysterious scenario where the cylinders flooded with water during a very windy and very fast reach across Georgia Strait. I can't imagine how it could have happened via the exhaust outlet, so I convinced myself that the prolonged speed pushed water through the strainer, up the inlet, past a worn vane on the raw water pump, and backfilled the exhaust manifold with sea water. It was easily cleared with the engine decompressed using the starter, but it probably didn't do the engine any favours. In any case it has never recurred so there is no real arguement for closing the inlet that I can see.