somewhere on the internetz, There is a little youtube video series about JK guys (Troutman and Dane) going to oahu and getting skunked looking for whitewater, and then finding some goods on the the big island. Awesome falls and lava channel rapids on the wet side. It looked very manageable and fun mostly....
There was also a mish in one of the older movies to run the Waimea Canyon (or maybe it was a story Oregonkayaking.net)
actually, here is an account from AW:
American Whitewater - Koai Camp to Waimea Canyon, Koaie Stream Hawaii, US
American Whitewater - HI State Rivers
again i got nothing for oahu, but on Kauai, we did a guided sit on top kayak day trip around the Napali coast, (19 miles with about 2-3' swells, it was super beautiful, but it was choppy and rowdy and could get too nasty in hurry, for us landlubbers.) we also paddled flat water up another river to hike to a hidden falls (Wailua river) it was very cool, but a pretty standard tourist trip as well...