wooden floorboards for boat

I need to make some new floorboards for my sailboat. They need to be light, durable, water resistant, around 3/8 in thick, and fairly inexpensive(Im in college. I like beer. Im poor). Any ideas for a good material to use?

forming grade plywood, its the stuff with the red paint on the edges. Its exterior glue, weighs less than treated and close to true marine grade as far as voids go. You may want to paint it ratehr than leaving it unfinished.