I've been once, and it was in July. I second the recommendation to reach out to Anthony. I did all my diving with Fathom Five (Anthony worked there when I went). We did 2 shore locations:
Koloa Landing: Right by the shop, super easy & mellow. Nothing too dramatic, but I did see a Dragon Moray here, which was awesome. Would be easy to do this on your own. Parking is easy!
Tunnels: North shore, apparently you can usually only dive this in summer. I loved these shore dives. Wish I had done more of them. Could do it on your own, but might be good to do a guided dive first. Lots of cool critters, great topography, and insanely gorgeous views as you swim out or surface and look back at the shore. Parking is hard!
I also was unsure about the Ni'ihua trip (both cost and I don't do that well on boats). But I thought - I might never have this chance again - so I did it. The accolades are all justified. It was really stunning. Sharks, big schools of reef fish, rays, and we got to dive with the endangered Hawaii'an Monk Seal for like 2 hours. I got violently sick the whole ride back (2.5 hrs). Probably the sickest I've ever been. 5-6ft. seas in a 6-pack boat. But it was worth it. They ran a very safe trip.
I did a couple boat dives on Turtle Bluffs & Sheraton Caverns. Sheraton in particular was cool, with little swim throughs to investigate, blacktips, and a couple of huge frogfish. But if I had to choose, I would do Tunnels over these boat dives.
Here are some pics:
https://flic.kr/s/aHsk2Y6afz
And a video from Ni'ihau (btw, the "seal whisperer" in this video is Anthony):
Enjoy your trip. Kauai is so lush and green. Save time for the Kalalau Trail in Napali coast - it's the most amazing 2 mile hike you'll ever do.