Hello Frank,
I belong to a yahoo discussion list called the CW Navy and Marine list or whatever. Anyway, below are a couple of answers from those on there that know. Makes sense to me.
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Dave,
I’m far from an expert and I’m just starting to get involved in naval reenacting but I have been told it was designed open to allow water to pass thru rather than hold water as the army types would tend to do.
John Stich

---John is correct. They were designed to NOT hold water. Remember, Sailors did not walk around wearing these things day after day as did soldiers. They were kept in lockers until needed, keeping them out of the weather. As soon as what ever evolution that they were issued for was complete, the pistol was returned to storage. There would be a Gunners Mate or on larger ships, an Armourer to maintain the weapons. Again, remember, these were not issued to any specific Sailor for the term of his enlistment as was done with Soldiers, but rather a tool that the average Sailor did not particularly care for.
Steve Hesson