Been to both, Grenada by cruise ship and Bonaire by land and cruise ship. Grenada is a great couple of tanks but that's about it. Bonaire is a great diving and surprisingly a great little downtown area with booming restaurants and nightlife. As great as that was, I don't understand why more people do not consider Hawaii.
In Oahu you have to the south, the SeaTiger wreck, to the West you have Electric Beach where the water appears neon blue with only sand on the bottom but you'll nearly guaranteed to see Rays and Large Turtles, to the North, you have the North Shore which is full of marine life and to the East, the china wall also full of marine life. You then can snorkel with sharks, can do so from in or out of a shark cage.
In the Big Island, you have Kona. If you're really really lucky, you could see the local celebrity tiger shark, Laveran. You have the night dive with Manta Rays and some of the best Pelagic/Blackwater (tethered to the boat at night when the pelagic marine life comes up to the near the surface to feed.
There are a lot of octopus on all islands, they are the "day" or the species which actively hunts in the daytime and its not uncommon to see them hunting/swimming going from rock to rock.
Hawaii should be faster and easier to get to for most people compared to a Caribbean Island and you're still in America. If you think hotels are too expensive look into rentals including those on airbnb
