I've been diving on the Big Island (Kona side), Lanai, and Maui. In Kona, my diving has been with Jack's Diving Locker and they do a nice job with the manta night dive, which I've done three times now. It is a spectacular show with the mantas doing barrel rolls right above your head. Usually you'll get 15-20 mantas coming in. It's become quite an attraction in Kona and there's a few careless companies that don't teach proper interaction with the mantas, and that is to say you should have NO interaction with them. Just sit on the sandy bottom and watch the magic happen. The reefs are bleached in some areas. For topside activities, you can visit and hike the volcano or visit a Kona coffee farm. If you're a coffee lover, this is a fun tour. Punalu'u Black Sand Beach is beautiful! You'll see resting turtles, which are protected (no touching or harassment) and the landscape is awesome!
I've been diving in the cathedrals on Lanai, but this dive is not for the beginner diver. The crossing from Maui to Lanai was 2 hours of rough seas each way...lots of seasickness on the boat. On another trip to Lanai, I took a snorkel tour. It's a shorter and more smooth ride and we had a pod of dolphins following us and the boat captain stopped a couple of times so we could enjoy the dolphins for a few minutes. I wasn't sure what to expect from the snorkeling, but it was actually really good on Lanai. You want to do the early morning tour for the calmest seas.
Maui is where you want to be in January because of the humpback whales. Whale season is December to April, but the best time to see a lot of whales is January and February and you'll want to stay on the west side of the island, like Lahaina or Kaanapali. The very best time to see the whales is in the morning when the seas are the calmest, and especially memorable is a sunrise whale watching tour. There's nothing like seeing a whale breach on water that is smooth as glass with the sunrise in the backdrop. There's so many whales in Maui that your head will be on a swivel. It's really quite an experience to hear the whale song while you're diving. It's a sound you'll never forget and it reverberates through your whole body. Molokini Crater is a great dive and snorkel spot. Again, the early morning dive is the best because the seas are calm. Molokini is full of sea life. On the dive boat trip to Molokini, which is about 45 minutes, you'll see lots of whales, and there's been times when our boat had to stop to let the whales pass. Another fun spot is Turtle Town, which is a location for a large aggregation of sea turtles.
If you're going to Hawaii and this is your first time, I would recommend Maui because the whales are seasonal. In Kona, the manta dive is done all year long. In general, Hawaii is not a bucket list location for diving, but I enjoy it for the variety of activities available besides diving.