The classic answer is swimmer's itch: flatworm larvae from snails, meant for waterfowl, burrow into your skin by mistake and die there - and your immune system raises the itchy allergic bumps on every water-exposed patch (skin under your swimsuit is typically spared).
It looks dramatic and is self-limited: resolves in days to a couple of weeks, not contagious, does not spread. Prevention: rinse and towel briskly right after swimming - dislodging larvae before they burrow is the real defense. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Why do I get a rash after swimming in lakes?
The classic answer is swimmer's itch - and it is a parasite story with a reassuring ending. The CDC explains it: microscopic larvae (cercariae) of a flatworm that cycles between snails and waterfowl get released into lake water; they are meant to burrow into a duck, but when they mistake you for the host, they burrow into your skin and die there - because you are the wrong host - and your immune system raises an allergic reaction to the invaders: tingling within minutes to hours, then itchy red bumps or small blisters on every patch of skin the water touched (a helpful tell: areas your swimsuit covered are usually spared). The important calibrations: it looks dramatic, itches intensely, and it is self-limited - the rash resolves on its own in days to a couple of weeks, you cannot catch it from another person, and it does not spread on your body. Care: rinse off and towel briskly right after lake swimming (dislodging larvae before they burrow is the real prevention), then cool compresses, calamine, or an antihistamine for the itch. The flags that mean it is not simple swimmer's itch: fever, spreading redness, or pus - that is infection and it wants a clinician.