The white plugs you see (opposite side of the hull from just aft of the shroud chain plate) are not vents or through the hull. They cover the molded-in inserts used for the tramp tension hooks on the Classic... each hull is molded with the ability to be a Classic or Club and port or starboard, so inserts are on both sides.

You can close the drain plugs while in storage. At least loosly to keep rain out.

There may be a small vent drilled on the inboard side of the rear crossbar sleeves on each side.

The caps under the seat pads have two functions. They are vents in the mold during the molding process and allow access to drop a second "shot" of material mid-cycle that creates the inner "foam" stiffening layer. The ports also allow us to reach in to add inner reinforcement plates for the tramp tracks. The caps use to be sealed only with hot melt glue, but now have screws holding them in. The hot melt does not give enough over time and sometimes allows a gap / leak. If the seat pads are not well glued... water can find it's way under and through the cap. You can reseal with 3M 5200 or similar. They have gotten better over the years, but Purple pads? Has to be pretty old now.

Seat pads fuzz off bits of color rather than simply leak dye I believe. Years of UV damage is the major issue.