The way you describe it you need to protect the wood from the water by adding an extra layer of glass first of all. That will eliminate the need for thicker plywood. I used 5mm hard plywood for my first boat, it was an indestructible tank, much stronger and heavier than necessary. Try 4mm soft or hard plywood with extra glass. The glass adds a remarkable amount of stiffness.

Henk

: I'm about to order wood for my first kayak (Wood Duck 12) and due to the many
: underwater logs and stumps in my area, I was thinking about using thicker
: plywood. The plans call 4mm ply, but I am thinking about using 6mm. Is
: this a good idea and will 6mm ply bend as much as 4mm?
: thanks