I want to strengthen my stomach muscles so I can get more quickness and strength getting up on my board, and to achieve better balance as I ride. I already do sit-ups. What else should I be doing?
Wow, kind of a loaded question as there are a ba-jillion types of core/abdominal work that one could be doing for speed, quickness, and agility. Well, maybe not a ba-jillion, but certainly a lot and not too easily explained over an email.
You could be doing functional and specific abdominal work for surfing using weights and bands. You could be doing static or moving sit-ups/core work using a ball, a bozu ball, bands, or weights. You could be doing pop-up actions while mainly focusing on utilizing your abdominals. You could be doing pilates for core strength, yoga, etc.
So, yes, there are ways other than your basic sit-ups to be strengthening your core for surfing (or any other sport for that matter). I would start looking at books, asking trainers, or checking online to see waht you can find. The core is super-important for not only protecting your back, but also for making you perform at a higher level of surfing.
Best of luck, and I hope this helps rather than confuses you!!