Fall is generally awesome time of year to head to Hawaii for diving. Water is usually it's warmest of the year from September 'til mid November or so, and the summer crowds are gone.
You'll get all sorts of recommendations as far as which island to go to. Each island is different, both above water and below, and has it's own thing going for it.
I'll give a plug for the Big Island as it's the one I'm most familiar with. The island itself is big, roughly twice the size of the other main islands combined, so there's lots of exploring to do. The island is largely undeveloped, the Kona (west) side, which is the primary diving side, you can probably count all the buildings taller than 4 stories on your fingers with fingers to spare, but still has the general tourist type of businesses people traveling look for.... restaurants, shopping, resorts, water and topside activities, etc. It does have a live volcano (you may or may not be able to walk right up to a live flow, depending on what's happening at the time) going for it as well.
The Big Island also has nice beaches, although being a geologically young island it won't be ringed with nice beaches like the others, you may have to seek them out some. Being geologically young has advantages when it comes to diving though... less sand, soil, and no streams or rivers (west side) makes for great conditions for healthy coral reef (
here's a NOAA article) with great viz. Reefs and dive sites are plentiful and boat rides are short unless you sign up for a long range trip.
Have fun!
