Replying to last post but commenting in general. Thanks again for all the replies and tips.
We found a VRBO on the Kona Country Club about 5 miles south of town. Should be a good location as a base to scuba, hit the beaches, travel up north and east, and enjoy the town and Ironman events yet still be able to escape that as well.
Also I downloaded the Hawai’i revealed App so thanks for that advice as well.
I have a house on the Big Island (near Waimea) but have also stayed all over the island (my wife is a travel writer). One thing about staying south of town is that the construction on the Queen K has created a traffic mess extending south and north from Kona. Keauhou, while lovely, often leaves you kind of trapped, so getting north is often not so easy. My personal preference these days is to either stay really close to the pier race week or to be far north (Waikoloa, Mauna Lani, Mauna Kea, my house now that my renters moved out), which lets you enjoy the great parts of the IM course and the island without the hassle of getting north through Kona every day. Race day you can get into town using the upper road.
Re: manta dives, the ones near the airport are also very good, and a number of companies go there - I like Kona Honu Divers, which is located in the same complex as Bike Works. But the diving is also really good north out of Kawaihae (Kohala Divers is the company to go with up there), especially if they’re going to Puako, which is sea turtle (honu) central.
The restaurant selection up north is also very good - I like it better than Kona. There is an Island Lava Java in Waikoloa Village, but there are many other great choices - check out Big Island Brewhaus in Waimea for fantastic beer and reasonably-priced food.
For riding, if you’re up there do the Kohala Mountain Rd ride to Hawi - that’s the last part of Day 2 of Ultraman. That is an adventure compared to the route IM uses, especially if the wind’s up. 
On the east side, find a VRBO in the tidepool area of Kapoho (south of Hilo). It’s on the famous Red Road, and it is the polar opposite of the west side. The nearby village of Pahoa has a couple of decent places to eat, and you’re a quick drive to where the lava is flowing and to Volcanoes National Park.
Ian