The summit of Kauai is Mt. Wai'Ale'Ale, which is the rainest place on the planet.
Our visit during March included rainy weather, of course, and made for interesting skies and wonderful lighting at times.

I carried a canon DSLR (XTI) and several lenses (17-55mm f2.8 IS, 85mm f1.8, 300mm f4 IS) dodging the rain, occasionally using a pack cover to protect my camera pack (Tamrac Velocity 9). Image stabilized lenses and a monopod were highly useful in the dim jungle under dark skies. I highly recommend hiking with a walking stick, or a monopod with a spiked foot (Gitzo Mountaineer monopod, Manfrotto #3232 head, RRS clamp). The trail along the Na Pali coast is notorious and our hiking sticks made all the difference, including fording the stream at Hanakapia.