The floorboards are covered by the class rules – download a copy from WIC;
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;
hope that helps