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Articles in Woodenboat and on their website fault Pressure-treated lumber for leaching poisonous salts into their surroundings, especially on boats where it can be wet for long periods. (No time to dry out and stabilize the poison -its always wet and always leaching out.) Beyond this, pressure-treated wood is often of lower-quality softwoods that soak up the poison better than stronger, harder woods. The poisons in it prevent decay, so it doesn't have to be as strong. Decks attached to a house are one thing, decks on a boat are something else. On a boat, marine ply is the best route, but I'd go with regular ply before I went with pressure treated in any case.