Local media say the 22-year-old was training for an upcoming Olympic qualifying tournament when she was his shortly after entering the water at El Tunco Beach, 10 miles south of capital San Salvador.
“Katherine embodied the joy and energy that make surfing so special and dear to us all, as a global ambassador of the sport,” the International Surfing Association (ISA) said in one of many tributes posted on social media.
Diaz had been preparing for the ISA World Surfing Games, which will be held from May 29 to June 6 at the La Bocana and El Sunzal beaches in El Salvador.
The top seven women at the event who have not already qualified will earn a place at the delayed Tokyo Olympics, where surfing will be a medal sport for the first time.
Additional reporting by Reuters.