I would go with the marine plywood in 3/4 inch thickness.
Then I would make sure my underlying supports are moved in closer to stop the flexing and get out the bounce.
Then I would get a good quality marine paint and mix in sand with the paint and use for your deck.
Sand makes a great non skid surface, and when it wears in spots, just add more.
Are your decks canted or at a fall to keep the water running back into the lake or whatever?
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