Niihau is fantastic. I was on Kauai last August, dove the south shore with Seasport via boat 3 days, did Niihau with them and also Koloa landing guided with them as well. I dove Tunnels on the north side with Fathom Five - no boat dives with them. Seasport runs a great operation, one of the best dive ops I've been underwater with.
The south shore diving was very nice, vis wasn't the greatest at 40-60' and there was consistent surge but the combination of I hadn't been Pacific diving before, Hawaii having so many unique / endemic critters and everything growing on volcanic structure is was very cool. Tons and tons of turtles, lots of life everywhere but some you really had to look for.
Niihau, on the other hand, was just off the charts outstanding. Definitely in my top 3 dives all time. Met at the shop at 6:30AM if I recall correctly, got on the boat and headed out by 7-7:30ish. Took a couple hours to get over there, dropped in on the first dive and my jaw just dropped. Endless visibility, no particulate in the water, no "dust" on the reef or critters, loads and loads and loads of all types of fish. Galapagos and a couple other types of sharks at about 120-150' perhaps deeper, we were at about 80-90' and it was so clear it was hard to tell how far away they were. Hawaiian monk seals on every dive. One one surface interval a pod of spinner dolphin stopped by and let us get in the water to snorkel with them and say hello. Lunch was basic with sandwiches and salads but really hit the spot. Plenty of room to move around on the boat, had deck chairs in case you wanted to try and nap on the way there or back. We were back in port by about 5:30PM or so. You can get back and forth faster with other shops, but the day is an adventure, not a race. I can't imagine that crossing in rougher seas on a 6-pack boat anyway. Between the crew, captain and diving it was an unforgettable day.
They start going to Niihau in "late spring" so hope the timing of your trip allows you to do this awesome dive! I purposely scheduled our trip so I could do this dive (site unseen) and obviously didn't regret it!
When we go back I'm going to schedule two Niihau dives it was that awesome. I'll for sure dive with SeaSport again as well. Great DMs, captains, boats, shop. They let you dive your own profile, provide excellent briefings and are very safety conscience.
Hope this helps!
