Aloha! Great inquiry.
While the majority of my experience is from diving Kauai/Niihau I have done numerous dives at Molokini seeing Gray Reef Sharks and a fair amount of dives (22) off Molokai and never missed the Hammerheads (though numbers varied greatly). Someone more up to date or on Maui can confirm - but I think Lahaina Divers and Extended Horizons both run weekly trips there in the summer.
As far as Kauai goes... a summer trip to Niihau is a must for those in search of sharks. I returned 3 days ago from a rare out of season trip across the channel and had 4 different species of sharks. The majority were Gray Reef Sharks with about 1/2 dozen Galapagos Sharks. A handful of whitetips were also encountered while one Hammerhead was seen. This was over the course of 3 dives and spearfishing did factor into the Hammerhead I am sure.
Without spearfishing you can certainly count on numerous Gray and Whitetip sharks on any given trip to Niihau. While not as common, I have been in the water there with Tigers, Sandbars, Hammers, and Blacktip Reefs as well.
Niihau is some of the very best diving in the island chain without debate. You are essentially assured to dive with endangered monk seals as well. Sightings of dolphins, large game fish, and rays are also very common.
Hope this helps!