Marine plywood - even though the name implies water, is not really a great product to be lying flat and collecting water - no wood is - I don't care how well you seal it - marine plywood does well if it's upright and allows for moisture to drain down the face.
I would recommend doing the deck in composite, there are several types, all with advantages and disadvantages, from research for doing my deck, I found it best if you buy a composite with 49% wood fiber, a light finish and rough surface, it will be comparable to wood WRT heat. If you buy composite that contains a higher percentage plastic or vinyl (worse) and has a smooth finish you will get more heat.
Finish has a lot to do with the heat transfer to your skin.