Galvanized nails can leave a nasty gash in a boat hull if they work their way out even a little bit. Instead, we've always used a 4 1/2-inch piece of fiberglass driveway marker rod. Available at Lowe's, Menard's, home Depot, etc. Cut to length with a chopsaw, bandsaw, or hacksaw - then drill a 1/4-inch diameter starter hole in the same place you would have put the nail.
Also, since plastic chain is so cheap, leave an extra half-dozen links below the buoy for a very easy way to adjust the buoy's depth to exactly what you prefer.
https://www.lowes.com/pd/Hillman-48-in-x-0-375-in-Reflective-Rod/3027086