Jewytim,
just recently fishing decking out my boat-- It was a 2 seater tiny -- i put casting deck in front and back-- use 18mm marine ply-- (make sure all cuts are seiled) on my last boat i just used plain 16mm ply and coated it with reson-- but alot easier as people have said to do it right the first time with marine grade! form ply is the next best and will probley do the trick, sheets are around 100 bucks though for a 3m by 1.2, As for any joints or fixing timber to ally use sikaflex marine-- stronge stuff and save scewing etc , also costs though at about 15-20bucks a tube.. but wont need much.
hope this has helped