From what I've seen, the $115-$125 most of the boat charters on the Big Island are in line with or lower priced than what I've seen on Maui or Kauai. There's some pretty low prices on Oahu if you look. The Big Island and Maui tend to have the best reputations for diving.
I'm partial to the Big Island, you'll probably find most posters here are partial to the island they live on or visit regularly.
By the way, I believe the Marriott is up at Anaeho'omalu Bay up in the Waikoloa Beach resorts area. Kiholo Bay is a semi-private bay off the highway a few miles south of there. Very picturesque. The majority of Big Island operators launch out of Honokohau Harbor about 4-5 miles south of the Kona airport, maybe a 30-35 minute drive from the Marriott. I think there's an operator that launches off one of the beaches near your hotel.
Aloha