I would hate to drill holes thru the hull, below the waterline. Can you use some galv steel brackets to hold the deck in place? They can be scewed to the deck and pop riveted or screwed to the seats with sheet metal screws.
Check out this link. The stringers are rivited to tabs that are rivited to the ribs. The decking is blind rivieted to the stringers and the edges of the deck sit on the the ends of the ribs. The deck is installed using half sheets to make it easier to cope and fit.
The last picture of the second set of pictures shows the best shot of the stringers from the front. They are 8' 1-1/2x1-1/2 x 1/8 aluminum. HD stocks it.
