Even tho' the British Standard 1088 is not enforced any more it is still quite common for ply makers to claim their product confroms to the standard BS1088. That's good because you can google for the 1088 stnadard and find out all the requirements it entails. So if your ply is claimed to be conformant to BS1088 that means it can only have ONE void across the face that is NO MORE than ½mm. That's .020 of an inch. If you ply had avoid bigger than that it violated the standard and you should have been able to get your money back.
Modern marine ply can be made on an assembly lines with so many sensors a pin prick would stop the entire line. Of course marine ply comes from around the world now and whose name is on the sheet can tell you something about who made it. A lot of no-name stuff comes from China and the prices ar good but the quality is iffy.
In terms of US made plywood there is very little quality control. There are no national standards. Makers belong to regional associations and they make and enforce their own rules. Since they can sell everything they make they don't enforce the rules very often.
With domestic ply there is nothing between 1/8" and ¼". The 1/8" is kind of flimsy and the ¼" is rather stiff. On a very short hull the ¼" may be hard to bend to shape. The alternative is 5.2 Luan which is generally poor quality and not recomended for immersion.
A quality BS1088 okoume ply in 4mm would be ideal. Expensive but much easier to work with. It should be plenty strong without a layer of epoxy/glass.

Hi Tyler
: I agree with Dave a sof may be better - he will soon grow out of
: whatever you make at that age
: Incidently Ive just finished a sof using 4mm pine ply for thr skin
: not even treated. It will be completely encased and only used
: for short bursts.
: Also re BS1088 - the pine ply I probably cut lengthwise 6 or 8
: times and never had a void but the BS1088 I used for the deck
: contained a couple of voids
: So much for standards
: Cheers
: Niven