While sanding I realized that they all have been right above the water line of the boat where the water constantly splashes. None below the waterline and none more than 4 inches or so above so perhaps there is a clue in there somewhere of the cause.
Very interesting! Not the usual pox pattern my old geezer brain recalls seeing "below the waterline" on many boats over the years. My first reaction is to think the source of the water is from inside the boat and it is being trapped at that level somehow. Just a thought. Good luck!