Aloha! Glad to hear you will be visiting Hawaii next year. Yes, we do see whalesharks! I would say that we tend to see more in the winter. But we have seen them year-round. Since there is not a predictable pattern, you shouldn't schedule your trip around the idea of seeing one. They aren't common...but it sure is a pleasant surprise when we see one. (We had a couple of years in a row with whaleshark sitings on New Year's day. A very nice way to start the year.)
My favorite months for diving here are summer/early fall (July-October). The water is at its warmest and the seas are generally very calm here in Kona. But we do have great diving all year.
We see mantas year-round. But, again, with calmer seas in the summer/fall, that is your best bet.
Not sure which island you plan to dive...but if you are coming to the Big Island, there are a ton of places to stay ranging from ultra expensive to "thrifty". The book, Hawaii: The Big Island Revealed" can point you in the right direction for accommodations. There are also books in the series for the other islands.
Let us know if you have specific questions about diving in Kona.
Mahalo, Angela