I've been scuba diving and snorkeling all over the world and I honesty think Kona is the best...but I'm sure many will disagree.
The beaches on the Big Island are not great but they are OK. They are not much better on Maui. Kauai has the best sandy beaches in my opinion. The farther east west you go the older the island. Older = more eroded = sandier. Maui and Big Island beaches tend to be rockier.
Fishing can be good or bad anywhere but keep in mind that the international billfish championships are in Kona.
Resorts are their own little world anywhere so I can't really say one island has better resorts than another in general but I will say that the cost of a similar resort will be double or triple on Maui compared to the Big Island.
Great hiking here as well in Hawaii Volcanos National Park (main park and also Kahuku unit) and the Kohala mountains as well as some rainforests.
Don't get me wrong, Kauai, Maui, and Oahu are all nice in their own way. Maui is having some difficulties right now. My recommendation for the Big Island is based on the activities you mentioned rather than because I live here. You can find all those things on every island, I just think the Big Island has the best collection of what you mentioned.