Rain water coming in through the inside of the mast?
I don''t know how much engineering that you are familiar with but here is how I am looking at this. I did discuss this with a couple folks that work at Farr (which are the folks who designed my boat.) While nothing was conclusive, they were in basic agreement with the logic on this.
To begin with, if, in its original keel stepped setup, the setion of the mast that is below the deck is considered as being an extra ''panel'' of the mast, which they usually are for design purposes, and which is why keel stepped masts are sometimes touted as being stronger, then the deck structure has to be designed to take thrust from the mast in all horizontal directions. Some decks are designed to take horizontal loading and some aren''t. Where the deck is not reinforced to take horizontal thrust then a keel stepped mast is actually significantly weaker than a deck stepped mast, but that is another story.
At least in the case of my boat, it would appear that the deck is intended to take horizontal loads as there are glassed in fore and aft and athwarthips bulkheads at the mast. So glassing in the hole in order to take horizontal loads is really pretty easy.
The next element of the design problem is the portion of the mast below the deck. No, it is not simplu acting in compression. If the top plate of the post below the deck (kingpost) is welded and gusseted to the kingpost, and the base of the mast is similarly weleded and gusseted so that a moment connection can be made on both sides, and then the plates are thru-bolted to each other through the deck, then a moment connection is made through the deck and the post below the deck will recieve some moment loadings. The base of the mast will then be a fixed end connection and so will in effect have the extra panel implied in a keel stepped mast.
So that''s the idea.