Throw in my vote for marine grade plywood. Especially being a permanent dock where weight isn't an issue. We have a steel roll in dock that is decked in 5/8" marine grade. Maybe 7 sheets. It ain't light, but we just replaced the wood 2 or 3 years ago. First time in my 30 years.
And the wood was still good, ie solid. Just the top layer was cracked and peeling in spots and more prone to slivers. Top of the wood was always stained every couple years with who knows what. Can't recall what we did with the old plywood but it is all gone and used on something else now.
As for water concerns, we are on the north side of a lake with mainly south winds so the bottom and top spend lots of time being exposed to water, rain, wind and sun. Plus however much snow falls on it each year.