One thing to keep in mind...a perfectly sealed composite mast will still accumulate water inside. This happens to all composites over time due to osmosis from humidity in the air and/or condensation of vapor in the air already inside the mast. I've seen near new carbon masts on Tornado's getting drain holes put in and teams draining water out before the mast goes on the boat for the first time.

Putting in some sort of drainage hole that can be capped is a good idea. The hole should be located between the upper & lower foam blocks, at the lower end of the largest open area in the mast.

One effective way to get water out of a sealed structure like a mast or centerboard/rudder...hold the item so the water pools over the drain hole. Blow into the hole to slightly pressurize the interior, then remove the pressuring device (ie your mouth!)...water will then spray out quick effectively. Repeat until done.