: For a 19' x 20" S&G kayak with a 200 lbs person in it, is 3 mm plywood
: feasible for hull and deck ? Let's assume I put 6 oz glass on both sides
: to strengthen it up.

Henk, 3mm will definitely keep you afloat. The question is how much stuff can you hit without punching holes in your boat? Here on the Texas Gulf Coast, I can build as light as I want with relative impunity, since one must travel at least 200 miles to find a genuine rock. And that's if you head inland. I feel perfectly confident building a hull with 3mm, and skipping the interior 'glass altogether except for the seams.

But if you must be able to withstand some amount of beating, might consider strip-building. The 1/4" thick core will serve you well, but won't hand you anywhere near the weight penalty that thicker plywood will.

Cheers,
Kurt Maurer