Hi there -- you can get a lot of information about getting married in Hawaii in general by checking out the "destination wedding" and "local Hawaii talk" boards at
www.theknot.com.
General consensus seems to be that the Big Island has the best diving, followed by Maui. But this is somewhat subjective, of course, and I've been diving on Kauai and wouldn't turn my nose up at it! Same goes for the other islands as well, I'm sure.
I don't think the Big Island is best for surfing, though, although others can probably weigh in on that topic.
Depending on your interests, you'll want to pick the island that best "suits your personality". Each has its own flavor -- some are more rural, some have more shopping and nightlife, some have more rain, etc, etc. There is a series of "Revealed" guidebooks (blue covers) that are particularly good and will give you a good overview of each island. You could spend an afternoon perusing them in a bookstore cafe...
My husband and I absolutely love the Big Island, and we were married there 3 years ago, at the Four Seasons Hualalai. We had 20 guests and it was fabulous. Within a year, I had friends get married on Maui, Oahu, and Kauai -- and everyone was very happy with their choice. After all, it is Hawaii -- it's hard to go wrong!
Happy planning!!