Ive never been to Kauai, but I have spent some time on Maui & I dont expect that much is different: very few boaters. From my experience, most of the rivers on the islands run when it rains & you have a very short window to put your run together. Personally, I have a passion for surfing, and if I were in your shoes I would surf every day.
There is a guy on Maui (sorry forgot his name check out Boatertakss Surf Zone for more info) that has some modern boats who takes boaters surfing for a living. Look him up. He is a local boater who is also a guide (no turf wars!); an early day or too with him may really pay off over your two-week stay
Try not. Do, or do not. There is no try.