If you are going to invest that amount of time and money in boat, why skimp on an item that might save you only a small percentage of the total cost but drastically reduce the value - and possibly the safety - of your boat?

You pays your money and you takes your chances.  The Pocket Ship was designed using materials specified in the plans.  If you use ply that does not measure up to the standard that marine grade Okoume meets, you might not have as strong a structure as you would like.  But it's your boat.  If you want to re-engineer it, and you don't have a suitable background, don't complain to the designer later if you are unhappy.  Good luck.

If you can find it, good quality exterior plywood would work fine for what you're talking about. Just keep in mind that most of the time exterior has a pretty lousy finish and that by the time you fill and sand it, it would have been a lot faster and pretty close to cheaper to go with marine.

I've seen people compromise by using okoume for hull & decks and meranti (which is still marine, but substantially cheaper) for bulkheads and such.

For that matter, okoume's main advantage over meranti is its lightness and flexibility. Neither of these is as important for the Pocketship as for kayaks. You could just build the whole thing from meranti. Use 1088 for the hull and deck and 6566 for interior parts. The typical prices from a supplier who sells all 3 are $83, $48 and $38 per sheet for 6mm Okoume BS1088, Meranti BS1088 and Meranti BS6566. Note that CLC sells the same sheet of Okoume BS1088 for $70, so they're the better deal on Okoume. CLC doesn't sell Meranti.

Laszlo