Vollmer began working closely with Milt Nelms, the so-called Water Whisperer, whose zen approach to the sport has developed a cult following, especially among Australians.

WSJ: 2012 London Olympics: Dana Vollmer Sets World Record As American Relay Team Loses to French

In a statement from its parent company, Discover Financial Services chairman and chief executive David Nelms said the company had "worked hard to earn the loyalty of our cardmembers, and we are committed to marketing our products responsibly".

BBC: Discover to refund $200m to customers