I recently took a lesson on the South Shore, which was fun and rewarding and I actually caught a wave and rode it myself two days later down at Poipu. Here's why I think it's a great sport and fitness pursuit:
You're getting super fit (over time) without expressly trying to. When I was done with a "session" (i.e., tumbling off the board, scrambling back on in the froth, and taking my licks), I felt like I'd done an all-body workout without thinking about it.
Different than the gym, when you're trying to build up those shoulders. Lots of rapid paddling, climbing up on the board in surf, even lugging the board around, for me, a heavy 10-foot longboard. As a newbie, of course, I wasted a lot of energy, similar to a skier trying to learn powder skiing for the first time.
It's a fantastic sport for kids; I saw the "keiki" dealing confidently with the ocean and snatching impressive rides. All the kids ("groms") were tan, healthy looking, and gave off a lot of cool determination. They weren't inside somewhere in front of a screen. Surfing fosters camaraderie, or at least it should. Everyone looks after each other.
You're outside under the sun, soaking up the natural vitamin D. Being a "haole," I protected myself with a 55-SPF stick, but I did get big sun doses. This was combined with all that rich Vitamin A from the island's papayas (a great source of A, C, and potassium; better than pineapple). A papaya is an example of a high-carb food (about 22 grams of sugar per large fruit) that's actually good because its loaded with nutrients.
You're out in Mother Nature dealing with the ocean, which in the longterm fosters a solid sense of the sea. Fitness shouldn't always involve manmade, indoor, and synthetic environments. Like that other "Last Great Place" in America, Montana (the brown bear), you're also developing a working knowledge of and healthy respect for a top-of-the-foodchain animal, the tiger shark. Nature isn't just a pretty video montage on Animal Planet anymore.
This sport was invented by the Hawaiians, after all, and comes with its own homespun wisdom. Not to mention the lingo. "Surfing isn't about thinking, which lawyers, doctors, and engineers have a problem with," said my teacher, a good guy named Chris who had none of the usual arrogance of the experienced. It reminds me of skiing, in which you are usually told, "you have to do A, B, and C...but don't think."
Get a lesson and then practice what you've learned until it becomes intuitive. Two musts that apply to other things in life: "Never turn your back on a wave... and watch a beach and the break for at least 10 minutes before surfing it."
I also spent a lot of time on Hanalei Bay on the North Shore stand-up paddle boarding, which was cool. Surfers call stand-up paddle boarders "janitors" and "moppers."