The Sandfather by Linda Newbery
Hal's life is in upheaval: trouble at school; problems at home; a quarrel with friends. Then Hal is sent to his grandfather's seaside home.
Soon, his anger is absorbed by the pull of the crashing waves, buffeted by the tides, smoothed by the endless sands. Yet thoughts of the father he has never known won't go away. In fact, soon his dad seems to be everywhere: in the sand figures Hal makes on the beach; in his dreams; in the shape of Wesley Prince, his mother's old boyfriend. Since no one will tell him the truth, Hal is determined to find out for himself. Will his elusive father ever come into his life? Or is he no more substantial than the sandfather, blurred and dissolved by the incoming tide?