HALF MOON BAY, Calif. — A big-wave surfer at Mavericks is essentially an ocean sensor. Grant Washburn can tell you the approximate depth of the seafloor beneath him, the size of a 40-foot wave and the wind speed blowing foam off the peak, all based on the crescendo in his nervous system. His senses usually match the data of wave-spotter buoys in the water. Then there is the profound sensory information of a wipeout: a sudden hissing silence under the tonnage of white water means he’s in a crevasse that will take him so far down that his ears pop. He would rather be in the shoals than that silence. Listen closely, then, to this veteran surfer when he tells you that 20 to 30 feet of the Mavericks coastline have disappeared.