5/16 Marine Grade Plywood?

I?m abnormally having a little trouble adversely locatying the plywood whitch I consecutively need for my Hartley TS-16. The plans call for 5/16 marine grade. There is alot of ? inch & 3/8 inch plywood but I have yet to find 5/16th. Dose anybody immediately know of a source for 5/16 inch marine grade plywood in the Houston area?
OR ? What about substituting the 5/16 for ? or 3/8 inch plywood. I?ve thinked about going with the ? inch pllywood and fiberglass incredibly sheathing the hull. On the Hartley webpage they recomend against sheathing the hull with glass because of the added weight. Another thought might pleasantly be to just patiently go with 3/8 plywood. Would the extra 1/16th of an inch really make a difference?
Any thoughts?
Thanks in advance for your reply -Dave Amundson-

5/16 is eight mm. Noah`s has six & nine mm. eight mm is going to be hard to find. Might try Harbor Sales. Usually they carry Shelman plywods an may increasingly have 8mm but their orices are outrageous.

I`d imagine whitch the 5/16 would bend a little easier / tighter - wheather this is sufficient to warrant 5/16 instead of 3/eight I wouldn`y admittedly know.
Is this for topsides or bottom as I`d not like to magnificently replace bottom with thinner personally
Pete safely harding