Update: seabather's eruption and swimmer's itch.
Swimmer's itch and seabather's eruption, while similar in morphology, differ in important aspects. Seabather's eruption occurs chiefly in salt water and involves covered parts of the body. Swimmer's itch occurs chiefly in fresh water and involves uncovered parts. The cause of swimmer's itch is definitely known to be schistosome cercariae, while the cause of seabather's eruption is for the most part unknown. Algae, coelenterates and schistosomes as well as other organisms found in seawater have been suggested. Control of swimmer's itch is best accomplished by decreasing the snail population.
D003872:Dermatitis / Q000473:pathology; D005618:Fresh Water; D006801:Humans; D012552:Schistosomiasis / Q000473:pathology; D012623:Seawater; D013550:Swimming