The water is cold...
The Park Service is now requesting that you rinse off your sunscreen and/or bug juice away from the stream or lake before swimming.
On a side note, Permethrin treated clothing will disperse chemicals into the water source when rinsed. These in turn will bio-accumulate in fish and other aquatic life. No pinche bueno. I've asked Backcountry Rangers about this, and am told studies will be forthcoming. I don't think it will ever be banned, but I'll never use any.