I have stayed at the Waikoloa a couple times. I booked the black water dive with that operator (can't remember the name, I think it is through Kohala dive shop), but went with Big Island Divers when a spot opened up. I have heard good things about them and it is very convenient. However, I have heard their boat is small, so you may want to ask about that. Especially if you are doing the blackwater dive.
I would also look into diving Puako. There are a few shore dives there that are fantastic. I rented gear from Kohala divers in the shopping center right next to the Waikoloa. I also dove with Jackson Seay, a personal guide, who was fantastic and he lives right near Puako. He can bring tanks to your dive too.
In my opinion, Place of Refuge (aka Two-step), a shore dive on the south side of the island was my favorite dive on the Big Island. I would definitely consider that too.
Sorry to get off subject a bit, but I hope I helped. PM if you have any additional questions.
