I'd definitely say try Maui one week and Kauai or Lanai the next week. There are so many great spots to snorkel or dive you could probably cover a few of the islands in one trip, depending upon your diving schedule.

If you do happen to visit Maui here are some of the top snorkeling and diving spots.
http://www.prideofmaui.com/blog/snorkeling-maui/top-5-snorkeling-spots-in-maui.html