My favorite dives to date while on Kauai was the Niihau trip (The Forbidden Island) through SeaSport Divers out of Poipu. It’s a long boat ride, but well worth it. Super rough on the way back so wear a patch. Spectacular diving. Best shot at diving with the near extinct Monk Seals.
My favorite dives to date while on Maui was the Cathedrals on Lanai. I think every dive shop goes there.