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Start the quest by interacting with the 'Swimming Test start' audio box on the northeast shore of Spruce Knob Lake (near the boarded-up shop). Put on a swimsuit before beginning the test. One can be found in a port-a-loo behind the shop.
Swim to all five buoys in the lake within five minutes and return to the starting point. Leaving the water or getting too far out of the water will result in failure. Mirelurks including a mirelurk queen live in the lake. They will spawn during the test if they are not killed before starting it.
PC The quest fails automatically if you swim beyond the furthermost buoy, even if you remain in the water. [verified]