You'd probably find enough boat diving and sightseeing on any of the four main islands to be happy for at least a week. For sheer number of developed tourist activities, Kauai clearly ranks below the other three.
For diving, I put Big Island (Kona area, or Kohala) on top by a wide margin, but that's because of the abundant high quality shore diving sites, and superior water and wildlife quality they possess. For a handful of boat diving trips, the advantage over Maui probably disappears, and Maui has the best beach and most beach lodging options. There are a couple of nice beaches on the Big Island but it's mostly lava coastline with some small sandy spots.
Both of those islands offer considerable top side attractions to keep you busy for quite awhile. The edge for variety probably goes to Maui, but the active volcano on the Big Island is a special wonder. For cramming the most into a short time, probably Maui is most conducive - you'll probably find more topside activities in close proximity to your lodging, esp if you stay down south in Kihei or Wailea. Kihei is more budget tourist condo, Wailea is pricey beach hotel. On the Big Island, Kona is more centrally located than Kohala where the top end resorts and beaches are. Some of the Big Island topside attractions, like Parker Ranch, Waipio Valley, the water falls, Hilo, the volcano, the black sand beach, tidepools, and the jade beach, are more day trips than an afternoon drive from the west side.
Oahu is iconic, much more developed and crowded, and doesn't seem to offer the same attraction for divers. I've only dived a handful of sites there, so I can't really comment comparatively.
