Man with boat (Planet of the Spiders)

A customer was testing a speedboat from the boat salesman Hopkins when Lupton, who was being pursued by the Third Doctor, stunned Hopkins and stole the speedboat and pushed him into the water. He made several attempts to get back on the speedboat, but Lupton finally succeeded in pushing him off. The customer eventually swam to shore. (TV: Planet of the Spiders)

Behind the scenes
The customer is named Pemberthy in the novelisation.