Dove with Fathom Five and they were a good op. As a matter of fact, the day we did a shore dive, they took a family of 6 diving.
A couple of things:
In April the road that gets you to Tunnels on the north side of the island was still closed. Whoever you dive with can tell you if it's open yet. Had hoped to shore dive and snorkel myself.
I asked several of the dive ops about water quality at Koloa Landing as I had been watching one of the websites that report water quality. Of course, they said there was no problem! If you went by the quality maps, you probably wouldn't get in the water several places in Kauai.
I posted a trip report regarding diving with Fathom Five here on SB. If you are diving or snorkeling Koloa Landing, be sure to take your dive boots or if snorkeling, some type of protective water shoe. To get out to where you would start to dive/snorkel, you have to walk through some rocks in the water that are golf ball to football size. I took full foot fins and no-one warned me about the entry - I jammed a toe so bad it turned purple for several days. If there is a slight surge coming in, that doesn't help either - my toe hurts right now thinking about it! Also there are 2 directions (east and west or left and right) you can go at the landing, so if diving with an op, it is a 2 tank dive.
Chad was our guide for the landing. His girlfriend told me that their favorite place to snorkel was a beach somewhere on the SW side, not far from Koloa. Said there was always something to see. You might check with Chad to find the name of it.
Although, the humpbacks were supposed to be gone when we were there, and surely will be when you are, we dove with a whale on our first boat dive. Awesome!!