Personally I would avoid hardwood simply because of the weight and the wear and tear on gear working with it. Marine plywood to the best of my knowledge is treated plywood.
If you have a mill nearby the best deals are often found there. My local mill turns out 7/8" pine on request, but usually waits until the orders add up enough to make it worth their while (say a few hundred to a few thousand feet). A lot of the pine when it was milled to the 7/8 was slightly rough on one side due to the rough lumber not being quite thick enough to allow sufficient planing on both sides. This never bothered me as I turned any rough pieces inside.
The beekeeping supply shop in Nova Scotia found out about the mill
and approached them to crank out big numbers (40,000). They now mill their rough lumber slightly thicker so that when they plane it both sides are smooth as a baby's butt. The last run of pine my buddy Kevin went to pick up (400 feet) was so nice he grabbed the last extra 200 feet. It was
really nice stuff.