I recall the one time I took the train to get from L.A. to San Diego. It was a very pleasant ride along the Southern California coast line, touching surf break upon surf break. Everywhere I’d see people getting into their black wetsuits by the trunks of their cars and running happy towards the waves. There, they’d join a multitude even on a workday morning, and all of them would bob on the waves, sitting on the board bored until a good wave would come. Then, one of them would jump and catch it, and all the other ones would wait. They couldn’t see anything, since they were all behind the wave, so it’s not much of a spectator sport, either.
The scene repeated itself along the entire coast. Boards whose nose poked out of the water, black swimsuits with bored-looking guys, only occasionally a wave and someone that would take it. Then, the paradox: most surfers you’d see were very fit, if not downright underweight. Let’s face it: they were mostly rail thin. How is it possible that a sport that looks about as physically demanding as watching the football game in the couch produces these skinny worshippers?
What do you know, I started surfing myself. So now I know, since every time I go out, I get my ass kicked again and come out of the water sweating (even though it’s a scant 60F). You see, surfing is very demanding when you don’t see it. Mostly, it’s because you spend a lot of energy moving your board when you are not on it: you have to get the surfboard to the right spot (the lineup), and you have to move left and right to catch the waves right. On top of that, there is the work you do to stay afloat and to turn to the waves.
By comparison, the surfing part itself is not much of a workout, mostly because it is so short. Even if you are a really experienced surfer, in most places a wave will carry you only for less than a minute. No matter what you do in that time frame, it’s just not enough to make you sweat.
Accordingly, the muscles you develop while surfing are not the ones needed to stand on a board (which would be the squatting muscles: glutes, quads, and calves), but the ones that you need to paddle: glutes, back, upper arm. From a workout perspective, surfing is more swimming with your butt clenched than standing on a board.