The rowboat owner was a toothless old Muggle. In 1991, he lent his rowboat to Vernon Dursley, so that the Dursleys and Harry Potter could get to the hut placed on a rock off the sea coast, and subsequently spend the night there. They needed to do so, because Vernon and Petunia Dursley wanted to escape the grasp of Hogwarts. But they were unsuccessful, as Rubeus Hagrid appeared and personally delivered Harry's letter. It is uncertain whether he returned for the Dursleys, or they returned to the mainland on some other transportation.