If you need any specific help with dive questions regarding Kauai feel free to send me a pm. You'll do yourself a favor (I feel) by staying on the south shore to avoid long trips to dive boats. Seasports and Fathom Five go out of Poipu. Favorite sites include Turtle Bluffs (slam dunk for sharks, octopus, large morays, turlte cleaning stations, and schooling fish), the lava tubes at Sheraton Caverns (interesting topography, loaded with turtles, and home usually to one or more frogfish), and Ice Box (Good spot for undulated morays, octopus, whitetips, bi-color anthias, and nudibranchs). If the weather cooperates the shore dive at Koloa Landing can be great as well (home to the rare Dragon Moray).
Hope your dive plans are coming all together. Niihau will be at the tail end of the season but I hope you get lucky and make the run.
Best of island wishes