The fishing is in the flats. There are a couple locations on Kauai. The problem is the trade wind. I don't do it myself. Have seen others with little success here. It is done by wading here rather than by boat. Some folks go out into deep water and fly fish for ahi (yellow fin tuna) from the boat with huge flies. Unfortunately, I get seasick.
The best flats fishing is on Molokai.