For all that you're looking for, I'd recommend the Captain Cook area, or someplace else closer to Kona. Kona has tons of restaurants and dive shops, and there are lots of good diving and snorkeling spots between Kona and Captain Cook, including several with shore entries. Not that Kohala (Puako) isn't nice, it's just a little more isolated. It's mostly resorts up there, and a shopping mall at Waikoloa. But the marina is just a few miles south, offering another clutch of dive shops. It's arid on Kohala and therefore the landscaping is very manicured, whereas Captain Cook is naturally lush (and has coffee farms - yum!). Perhaps you know all this already, and perhaps Kohala is what you prefer. Just my two cents.
PS -- if you're going there in the winter, then Kohala is a better bet as far as consistently sunny weather, not that Kona gets toooo much rain.