Billy Selway was the childhood best friend of Danny Williams.
The Williams family used to rent a summer house in Wildwood for vacations, and one year they were accompanied by Billy. The two boys went swimming, and he drowned while attempting to help Danny, who was caught in a riptide. This leads to Danny's fear of the ocean.
Steve McGarrett and Danny drink a toast to him at the end of Lana I Ka Moana