I've done a lot of diving in Hawaii and I think Kauai has the best and worst diving in the islands. The in-shore dives on Kauai are frequently sub par compared to the other islands. Generally speaking the viz is worse, the water is colder and the density of life is less. Mana Crack and Niihau/Lehua are the opposite, these sites are head and shoulders above the rest of Kauai and easily in the same class as some of the other top flight dives in Hawaii.

To paint you a picture: my favorite dive at Lehua is called "Vertical Awareness". You drop down on a pinnacle that's off shore from Lehua. This is in maybe 10-15m of water. Sharks are everywhere, bigger ones than you see in shore, mostly Galapagos and Sandbar. At first you point them out to the rest of the divers, after a while you stop bothering. You swim off the side of the pinnacle and sort of spiral down around it, you see tons of fish, the usual suspects of course, but also things like Yellowfin Soldierfish and Tinker's Butterfly! The water has almost no particulate and you feel like the visibility is unlimited. You're down past 30m now and you swear you can see the bottom down at maybe 70 or 80m. There are plenty of cracks to peek in if you want to go small, or you can swim off the pinnacle a bit and get a wider view. After a bit the DM signals that it's time to go up a bit, so you ascend to 20-25m and, taking a compass heading, kick off the wall into pure blue ocean. There's no visual reference so you keep a close eye on your depth and the rest of the group, but not so close that you miss the manta or the school of barracuda. After a time you see a wall loom out of the distance, and as you get close the current starts to pick up and you spend the rest of the dive drifting along the wall of Lehua crater, listening to whalesong, hoping to see a Monk Seal.