I know the night dive with manta rays is in Kona, can this be done during a day trip from one of the other islands?
No it's too far. Kona of course being the only area to reliably see Mantas although they do show up off Maui occasionally. It's also Volcanoes Nat'l Park and Kona town is pleasant but shore dive sites are a long drive apart. Most of the boats except for 3 in the Waikoloa area are based in Honokahau harbor located between the airport and Kona/Kailua town. Waikoloa is about a 1/2 hr. drive north.
There's a lot of waterfront - not beachfront condos south of Kona also. vrbo.com lists a bunch by area. For mom while you've got the car diving there's a trolley that runs between Keahou to the south and Kona town. We've used Kona Diving Company - you'll see them recommended here often.
I mostly agree with what Bubbles ^ posted but my choice would be Maui first as you can dive 3 other islands from there daily - Molokini Crater, Lana'i, the entire West Maui coast line (esp. shore diving) and Lahaina Divers even goes to Molokai to dive with Hammerheads certain times of year.
There's also the most shallow shore dives for the g/f. Kapalua Beach, Black Rock or any of the Kihei/Wailea beaches are also good shore dives - when you price boat dives you'll see why it's a good option.
Scuba Shore Diving Region: Hawaiian Islands
Kihei has a lot of condos in all price ranges. Often cheaper since many are across the street from the beach. Wailea is a little more upscale, nicer shopping resorts like the Four Seasons and many of them are beachfront. We rent condos there and walk to the beach w/o leaving the property. It's also a dive.
In West Maui there's properties in Lahaina - walking distance to the shops/waterfront is a bonus - or Ka'anapali/Kahana where the mega-condo towers are. It's nice there also on the beach but there's only one or two nearby areas with decent shore dives. The boardwalk is a nice area with a mix of restaurants/shops/small malls.
Still farther north is Napili/Kapalua. Napili is older condos on the water, Kapalua famous for one of the nicest beaches in Hawaii and the golf courses behind it. About a 20minute drive back to Lahaina for most things although Kapalua Beach is also a nice dive.
Kauai is good diving also - the best of it is Ni'ihau which is diveable in August but a rough crossing and all-day trip. On the list I linked to Tunnels is the best shore dive, Koloa Landing second. Ahukini is closed and many of the others better snorkels as they're shallow for a long way out. If you wanted to base in an area with good diving, access to diveops/boats and nice beaches - look for something in Poipu. We rented a big house (family trip) so I can't help with specifics.
hth,
