I unfairly indicted the Ducks, Geese, and Loons in my above post. Here is an article splaining this "parasite". It doesn't come from wildfowl poop.

NB
http://en.wikipedia.org/wiki/Duck_Itch
PS: I have found the procedure mentioned at the end of the article, (and below) also to be quite effective after encountering jellyfish in salt water. It seems to prevent any reaction to jellyfish stings.
...."periodic brisk toweling of the extremities is effective in preventing Swimmer's Itch".
I do this while still IN the water, and while emerging from the water to
Rinse the parisites off the skin before they penetrate.