At one stage sealed masts were a requirement for ANY "off the beach cat" to be permitted to sail at any club here in South Australia. No sealed mast, illegal to race! that was through the 70's, and thank goodness the yachting authorities changed that in their wisdom to "all catamarans racing shall have the head of their masts sealed and the base of their mast vented (able to drain), which is infinitely safer than having the complete mast sealed. THERE IS NO SUCH THING as a mast that is "sealed" remaining sealed. The danger occurs when any part of the mast that has a fitting or a penetration through the wall of that mast, will EVENTUALLY NOT BE AIR AND/OR WATER TIGHT, the when/if that mast is submerged due to the increased water pressure (opposed to air pressure) it will fill with water very quickly, making it very difficult to raise, and when/if it is raised, the water pressure that pumped it full of water is gone and it remains full or drains only very slowly.
The safest mast is one that is vented at both head and at the base