Aloha Mark,
There are two dive ops that dive Ni'ihau year round (Explore Kauai Scuba and Bubbles Below). We (Explore Kauai Scuba) will dive Ni'ihau and Mana Crack(Na Pali Coast) year round, weather depending. If the north swell is up, but the wind is down, we'll dive South Point Ni'ihau. We...