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The coolness of water will make a swimmer less aware of a growing sunburn, while reflection of sunlight is more apparent on the beach, it can occur in the water as well. The American Academy of Dermatology recommends reapplying sunscreen at least every two hours to remain protected or immediately after a swim or excessive sweating occurs. For long-distance swimmers, re-applying sunscreen can be impossible once it wears off, so using an alternative like Balmex can be necessary, said Wilson. Traditionally used to treat diaper rash, Wilson said the key ingredient of zinc oxide makes Balmex a useful sunscreen. Its stickiness and adherence to the skin for lengthy periods provides swimmers with adequate sun protection, she said. Exposure to bacteria, parasites Swimmers may be unaware of the dangers of toxic algae, bacterial growth and parasites that thrive in warmer waters. There's also an extremely rare risk of exposure to Naegleria fowleri, a deadly brain-eating amoeba found in bodies of warm freshwater.