I agree that Niihau has some of Hawaii's best diving, but getting there can be dependant on the weather. If the weather allows, GO! Explore Kauai Scuba, Fathom Five, Seasport, and Bubbles Below offer Niihau dive trips, all great dive shops!
If Niihau conditions don't allow a safe crossing, I recommend diving "Mana Crack" off the Na Pali Coast. I own "Explore Kauai Scuba" and we are offering almost daily trips up the coast. We are owner operated with a couple extra captains that have grown up cruising Na Pali Coast since diapers. We know that coast line inside and out. We leave from "Kikiaola Small Boat Harbor" which is the closest to Mana Crack and Niihau. Our surface intervals include checking out the coasts sea caves and steep cliff lines. I grew up with a geologist dad, so we don't just brief heavily on the amazing marine life, but also the geological history and how Mana Crack came to be. Mana Crack in the summer can offer some of the best visibility around Kauai. Since it's Hawaii's furthest offshore dive site, you consistently get 100+ feet of visibility. It does tend to be more on the advanced side due of currents. We keep our diver maximum at 6 divers, but on Mana, have been just taking 4 divers for the best experience.
Bubbles Below also dives Mana Crack. The new owner Clay is a really nice guy with a similar passion to Kauai diving, you can't go wrong! I worked for Fathom Five Divers for a handful of years and they're another 6 pack operator doing awesome south side dives and Niihau twice a week during the summer. Seasport Divers has the nicest boats but tend to take more divers, but still a great operation as well!