Best Hawaiian Island/Marriott Property for Diving???

[diversteve]

start out in Maui for 3 nights (no diving there unfortunately)

Sure you can't sneak in a shore dive or two? Drive is over to Kihei, rent tanks from Maui Dreams Dive Co. Wailea Beach, Polo Beach or Ulua Beach are easy entries nearby - there's others but the logistics of getting tanks then getting there might take more time. http://www.shorediving.com/Earth/Hawaii/Maui/index.htm

Do I need to worry about elevation if we head to Volcano Nat'l Park at the end of the trip or should I plan on doing this at the beginning?

Yes...

Kīlauea's summit (4000' elevation)

Also you might want at least a sweater at Mauna Kea where it's currently 5^o (Celsius) mid-day.
Mauna Kea Weather Center Forecast

Should you be doing a lot of driving cross-island the Saddle Road is 6632' also. From Wikipedia:

Route 200, known locally as Saddle Road, traverses the width of the Island of Hawaiʻi, from downtown Hilo to its junction with Hawaii Route 190 near Waimea. The road was considered one of the most dangerous paved roads in the state, with many one-lane bridges and areas of marginally maintained pavement. Most of the road has now been repaved, and major parts have new re-alignments to modern standards. The highway reaches a maximum elevation of 6,632 feet (2,021 m) and is subject to fog and low visibility. Many rental car companies used to prohibit use of their cars on Saddle Road, but some now permit use of the road. The highway experiences heavy use as it provides the shortest driving route from Hilo to Kailua-Kona and access to the slopes of Mauna Loa and the Mauna Kea Observatories.

 

[UnderwaterBumbleBee]

No, unfortunately diving in Maui is pretty much out. We're only going to have two full days there and we want to see Haleakala and road to Hana. With lack of time and heeding the elevation warnings (especially since we'll be flying to Kona and visiting Mauna Kea & Volcano Natl' Park) we just can't squeeze it in. I was only able to book 3 days of diving on Kona with about 9 dives total. It's definitely going to be trip on a timeline, I don't necessarily mind that though, I can lay around the house whenever I want. Got things to do and places to see!