Use 3 inch galvanized spiral nails for the decking.
For decking, we now again use 1 X 6 boards instead of plywood. The half inch plywood did not stand up well for us and we have put boards over top of it since these pictures were taken. The plywood was an attempt to save construction time.
It is of course possible to paint plywood so that it will withstand the weather, but we have no experience with that. Also, for a price, there is probably better plywood available than the ordinary building grade that we used.
Disregard the plywood unless you are willing to paint it to keep the moisture out or are a millionaire and can afford marine plywood. We have used half inch building grade plywood on several docks and didn't like the way it aged.
Now square the dock. Use the 6 - 8 - 10 method. Measure from each corner 6 feet one way and 8 feet the other way and make pencil marks at those spots. Then measure across the corner from mark to mark. The distance should be 10 feet. If it is not exactly 10 feet, pry the corner of the dock one way or the other.
To make adjustments, use a long crowbar as seen in this picture. Pry only on the corners, where the steel protects the wood. Move the corners this or that way until you have 10 feet from mark to mark. Don't be too fussy. We are not building a piano.
Use 3 inch galvanized spiral nails to nail the decking boards. Nail each board with two nails at every floor joist.