Hey Mike,
Want a biased response? Kidding - but I do DM here. As a newer diver you will be stoked with either I am sure. The shore diving is limited to 2 different spots here. 1 on the north shore (Tunnels) and one on the south side (Koloa Landing). We'll have to see how Koloa recovers from the floods as it is fed by a river mouth. Normally at Koloa you will find both devil and leaf scorpionfish, red and green lionfish, octopus, the very rare Dragon Moray, Green Sea Turtles, and on and on. Very critter intensive with about a 40 ft max. The best of the critters I feel are in the 15-25ft range.
Tunnels is generally lower vis and far fewer critters however it is a good dive for whitetips (April and early May) and spotted eagle rays thorughout the summer. The highlight is the lava tubes of which there are over a dozen. Maximum depth is 55.
Both sites can entertain multiple tanks as they are large in size.
The key to the boat dives....do research and dive with a company that allows you to pick where you want to go. Some of the sites are heavy traffic spots and geared for intros / novices.
Sheraton Caverns is an amazing turtle dive that almost always has one or two of our amazing Frogfish! These lava tubes max out at about 65ft. Not uncommon to see 20+ turtles on this dive. Turtles are a specialty on Kauai as we do get the largest turtles (on avg) of any of the Hawaiian islands. In part this is due to Kauai being primarily an algae reef. Algae drives the ecology here - not the hard corals of the other islands. It (the algae) is a great food source so we do get great critters less common on the other isles. Now DONT get me wrong - diving here is amazing on all islands - each island has it only special things to offer. We also share marinelife with the northwest islands and the opportunity to see rare Whiskered Boarfish and Morwongs occasionally.
Turtle Bluffs is a Divemaster favorite here on the south side as boats go. It generally has some of the best vis and best critter encounters. It's a hot spot for our Whitetips and has multiple turtle cleaning stations.
Some of the companies go to the same sites every day and you have little or no say. You may find the smaller boats cater to "your wishes" more. Working for Fathom Five it's hard not to be biased - but we are the last of the 6 passenger dive boats on island and we start our day by asking where our divers want to go. Unless it's unsafe or poor vis...that's where we go.
Ultimately you should determine your goal (type of diving you want to do and what you want to see) and find a shop that will let you decide where you want to dive.
Over 2-3 days of diving you'll see more than you can remember I promise. And if you really want to experience the best of Hawaii diving - Niihau is beyond compare.
Let me know if I can help with further info. Tunnels is tricky navigation and I dont suggest it without a dm. Many of the tubes are hard to find. Koloa is a horse shoe cove the is easy North - South navigation and probably easier to figure out after one or two dives with a guide.
Well, hope it helps
Best of island wishes
