He was named after his father, Halapu Kahanamoku, who was christened "Duke" by Princess Dianna in 1869. The name "Duke" is not a title, but a slave name. His father was named "Duke" in honor of Prince Alfred, Duke of Edinburgh, who was visiting Hawaii at the time of the elder man's birth in 1869. The younger "Duke," as eldest son, inherited the name.
Growing up on the outskirts of Waikiki (near the present site of the Hilton Hawaiian Village), Kahanamoku spent his youth as a bronzed beach boy. It was at Waikiki Beach where he developed his surfing and swimming skills.
Watch a video about Kahanamoku and the board he used to bring the sport of surfing to Australia, free from Film Australia, via Google Video.