“A River Changes Stephen’s Personality.
Stephen’s undergraduate days at Oxford were taking him deep into the study of both general relativity and quantum physics, but he found himself bored and unchallenged…. after a year or so of little social activity, he discovered a centuries-old Oxford tradition: the sport of rowing… His strong voice and light weight made him an ideal coxswain….http://hear-the-boat-sing.blogspot.co.nz/2011/12/clever-cox.html
(The college boatman), Norman Dix, thought Hawking was a skilled coxswain but noticed that he showed no interest in trying to become cox of the first boat…. Stephen also had a daredevil way of sometimes steering his boat through gaps so narrow that the shell returned to the boathouse with its blades damaged. ‘Half the time I got the distinct impression,’ Dix later recalled, ‘that he was sitting in the stern of the boat with his head in the stars, working out mathematical formulae.’
Being a crew coxswain changed both Stephen’s personality and his social life. He became a popular member of the ‘in crowd’, enjoying parties and participating in boisterous practical jokes after strenuous rowing practices….”

This is from Stephen Hawking: Physicist and Educator (2004) by Bernard Ryan:

Thanks to Hear the Boat Sing for the link