In fact, they can keep your boat drier than without. My Nordkapp has very well sealed hatches, but I always had a tiny amount of water in the fore and aft compartments after a trip. I also noticed that the hatches would be sucked inward as the cold water cooled the air in the compartments. I installed breathers to cure the latter problem and haven't had a drop of water in the boat since. I suspect that water was literally being sucked into the airtight compartments.

The way I do breathers is to use the red plastic tubing that comes with spray lubes. Drill a hole that fits the tubing tightly, insert it into the bulkhead, then trim it off flush with a chisel. You now have a tiny opening that won't readily pass water, but it allows enough air through to equalize pressure. If you want to use Gore Tex or something else over it, by all means do so, but I really think it's a waste of time, effort and money.

BTW, I drill the holes 1" above the center of the bulkheads. It's an arbitrary position, but it's high enough to keep them above water with the amount that I typically get in the cockpit during rescue practice.