Be careful and get good beta about anything you do on any of the islands. Lava tubes are kayakers worst nightmare. Lots of them on a lot of rivers that look very inviting, until you figure out that half that CFS just went underground and might not come out until it hits the ocean.
BTW, you might want to check with Kayak Kauai for local beta. Don't think they do any whitewater but a good source of info for that island on a variety of subjects such as kayaking, hiking and biking. If anyone knows local boaters on that island these guys might.