The floorboards are covered by the class rules – download a copy from WIC;
http://www.wayfarer-international.org/WIC/ClassRules_index.htm
This specifies a minimum thickness 8mm or 5/16″.
Whether you choose marine ply or not depends on your budget – Robbins will deliver marine ply. Better quality ply will also be stronger for a given thickness.
Personally I would always have “2 long sections”.
There is quite a detailed post on floorboards here;
http://wayfarer.org.uk/forum/viewtopic.php?t=355
hope that helps