When you life is at stake, one's faculties are sharpened. Back in the 1980s a very old coastal merchant navy skipper of my acquaintance was known to navigate around the North Sea largely by the shape, direction and frequency of the swell. He once woke from sleep in his cabin, sure that the ship was in the wrong place. He was right, and got to the wheelhouse in time to avoid grounding on a sandbar. The officer of the watch had missed seeing the buoy at which he should have altered course.