It is a vent, but they do have a tendency to get clogged. It's usually not necessary to clear them if you leave the plugs out when the boat's not the water.

My '98 H-16 has held air pressure since the day I bought it new. I never leave the plugs in unless I'm sailing it. The Tiger doesn't have vents, but it does have a warning sticker on the transom saying that you need to remove the plugs when it's not in the water to prevent pressure build-up.