Now, I'm only a Newbie to Kitesurfing and know stuff all about it at the moment. In fact I can hardly get up on the board at the mo, But I am a Personal Trainer and have been in the fitness industry for years. Prob longer than most of you youngsters have been kiting. LOL
AND No amount of "Sit-Ups" will harden your ABS, Sit-ups are ONLY good for your EGO!!!!

You need to be doing "CRUNCHES" and variations of them. Your Abdominals can be trained every day, as they are the quickest recovering muscle in the body. BTW the abdominals are 1 muscle NOT upper Or lower abdominals as most people would think. It's just that when you work your abs you actually recruit muscle fibres at different stages of an exercise, which gives you the impression you are working upper or lower.

Anyway the moral of this story is: If you are getting sore abs, "GO WITH IT" that just means that you are working muscles you haven't worked before.
Your body will adjust and you'll have great looking abs as a result. That is of course if you don't have a layer of FAT covering them. HaHa!