Swamp,
I used 2x3s for frame support (I think 2x4s are overkill, some would argue that 2x3s are overkill); 3/4 ply for decking; and 1/4 ply for walls. You can see from the pictures that it still isn't drafting too much water. I was impressed with the stability, rocking the boat back and forth (knock on wood) it would be extremely difficult to swamp it. If in doubt, I suggest doing the math:
The bouyancy provided is equal to the weight of the water displaced, I conservatively used 60lbs/sqft (quick internet search says 62.4) of displaced water in my calculations. So then it comes down to how much do you want your boat to draft and how many cubic feet are displaced at that level of draft. To simplify it figure out the square footage of the bottom of your boat (that portion that will be under the water); lets say you come up with 36 feet (12x3); if I am willing to draft 6 inches then I need two square feet to draft one cubic foot; to only draft 4 inches I would need 3 square feet for each cubic foot, etc. Now that everyone knows how much of a nerd that I am... three arguments for going full nerd; 1. Work smarter not harder; the alternative is that an aversion to math requires putting all of your building materials or equivilent weight and doing a test float to predetermine the outcome 2. I didn't want to be surprised about the flotation/draft/stability the first day that I took her out 3. All those years of sitting painfully through math classes have got to be worth something.
By the way, nice spoonbill...
Hope that helps,
DD