1 to 1 1/2 hour boat ride from Kaui.  Dive sites are breathtaking drop-offs, gigantic sea arches, canyons and caverns. 
Facts about Vertical Awareness - Nihau, Hawaii
 It is in United States- Vertical Awareness - Nihau, Hawaii is in the Pacific.
 - The typical depth is 0-40 Metres 0-130 Feet.
 - The typical visibility is 10-30 Metres 30-100 Feet.