Swimmers' itch, which can also be called 'dark itch', 'cercarial dermatitis' and 'schistosome cercarail dermatitis', is an immune reaction which is visible on the skin after an infection of the water based 'schistosomatidae'. This results in raised papules which are itchy but self limiting, disappearing normally after no more than a week. It is not contagious and generally not severe. It is called swimmers' itch because those who swim regularly in fresh water or sea water are more likely to experience it regularly.
The parasite schistosomatidae is generally released through faeces where it will affect an aquatic snail. The snail will then release larvae looking for a host, which will normally be a human or another mammal. When it find a human it will burrow beneath the skin and then die, and it then causes the immune response – essentially an allergic reaction. The rash then is caused by the body trying to force the infection out of the body, not by the infection itself.
However there are some other forms of swimmers' itch, and many people will use the term more generally to refer to a range of itches that an individual can get after swimming. For instance it is caused by another bacteria too bacteria too, called 'trematode nematocysts'. Meanwhile swimmers' itch might also come from trematode parasites. In some cases it might be a result of contact with jellyfish, sea anenome or other sea creatures. It might also be a result of an allergic reaction specific to the individual. In most cases it will clear up, but you can speed up the recovery by bathing in Epsom salts.