Dan Murphy, 21, shown instructing Mrs. C. L. Christiansen of Brooklyn on Lake George, N.Y., has made a profitable business of water skiing instruction in four short years. Now a senior at the University of North Carolina majoring in history, Murphy started skiing himself on Long Island Sound when he was 14. After graduating from high school, Dan borrowed enough money to open a water ski school on Lake George, has since opened another one there and two more at Westhampton Beach and Sayville on Long Island. Dan employs eight instructors in his school, guarantees to teach the rudiments of the sport in one lesson. He estimates that his schools will teach 2,000 persons this year. In addition to teaching, he has made movie shorts on water skiing, tours with his own traveling stunt troupe.