1/2". Anything thinner is too flimsy and you'll have a harder time with nutsert or hurricane nut installation. 1/2" you could even tap. Search Amazon for marine board.
Take a peek at my build thread. I just went over the same topic. 1/2" material 12x24 is $23 on amazon and high quality. I ordered 10 sheets for a discount. I tried 1/4" material but was not satisfied with it.