A couple of years ago, The New York Times published an article on new competitive surfing research.
"Perhaps most surprising for people who think surfing looks calm and meditative, surfers’ heart rates soared to a chest-burning 190-plus beats per minute during the competitions and rarely dropped below 120 beats per minute. The surfers also covered considerable territory while paddling, averaging more than half a mile during each heat, or about a mile and a half per competition."
"In other words, surfing is a considerable workout requiring high-level aerobic endurance, Mr. Farley says, given that the heart rate stays above 120 beats per minute at least 80 percent of the time, and given the amount of time spent paddling."
Sources