We like to snorkel, and have children, so for us we find that the winter months have too big of waves to make the ocean "usable" in many areas (for either swimming or snorkelling) and the ocean feels quite a bit cooler (still certainly not cold, but like the difference in the water temperature in Miami between January and July).
You're more likely to hit rain in the winter months.
In the summer, you WON'T see whales and you won't see surfers on the really big waves.
Your chances of having good weather at any given time of year are probably better in Hawaii than most places. But like anywhere else, the weather can still prove to be unpredicatable.
Where are you coming from? If you're coming from Texas or southern California, your need for sunny & dry might be quite different than those of us hailing from the upper midwest or Seattle!
It sounds to me like hot, dry, sunny weather is what you really, really want (I'm with you on this one!). I would aim for July or August. Staying on the south or west side of an island will make you happier (for example, on Kauai, go for Poipu rather than Princeville or Lihue. On Hawaii, go for Kailua-Kona rather than Hilo. On Maui, head to Wailea/Kihei or Kaanapali/Lahaina rather than Hana).