Almost all the inter-island flights pressurize the cabin or fly to elevation so it's hard to do a lot of different island diving in a typical week vacation.
Here's what I've either personally observed or my regular buddy has.
Manta's are almost always good on the Big Island, on one of the south of Kona dayboats spinner dolphins might be your buddies. Since your husband snorkels, he can do that at the Manta Night Dive - some think it's better since the boats have lights shining down so it brings the Mantas up.
Off Maui there's all the diving at Molokini Crater - a slim possibility for Mantas but likely a lot of sharks. One site is named Shark Condos. Off the Backwall (more advanced diving) your buddy might be a reef shark or two feeding on the wall nearby. It's a bluewater dive - about 300' to the bottom.
I followed a couple reef sharks there from about 40' into deeper water, they'd pause till I got closer then move again. You should hear Humpbacks singing - it reverberates off Molokini - a friend was lucky enough to dive with a baby Humpback there. - that never happens.
A lof of West Maui dives are generically called Turtle Town by one of the operators so that's likely. At one of the wrecks, as of a few years ago, they've taken over. My buddy even had one swim up to greet him. There's also a Manta feeding station nearby off Umekahame (sp). Late one afternoon my buddy saw two there on an Ed Robinson's advanced dive but Kona is a better possibility.
We've relibly seen white/black tips off Lana'i - not every dive but pretty close. Once off Sharkfin (hint) there were at least a dozen patrolling around the edges of a huge school of Morish Idols, Tangs and Butterflies.
Off Kauai there's always big turtles at Sheraton Caverns - a fairly shallow dive. It's really known for them. I saw at least 5 including one that must've been 1/2 ton or more - about 6-8' long. If you get there glance up, occasionally one circles around after divers disturb them.
There's one endangered Monk Seal that lives off Ni'ihau which is an all-day rough water crossing that should be possible in late April. But don't take your husband, there won't be anything for him to do. There's also some at the Waikiki Aquarium - a nice small place to visit if you're there. They also have Chambered Nautilus in a small tank inside - only place I've ever seen one.
Haven't dove Oahu.
Does your husband golf? He won't be bored while you're out diving. Kapalua Ocean Course on Maui is especially good. The Wailea courses are excelent also.
