If you are planning on carrying people or jsut want a bit more capacity, build with 2 inch thick foam sheets instead of the 1 inch thick stuff. Obviously it will offer twice the displacement. That give you a bit more reserve for carrying adult sized loads.

To make pontoons from 2 inch thick foam you'll need to make fewer cuts, and you'll have fewer pieces to join.

From each 4x8 panel you'll cut 8 foot long strips of various widths.

Try this cutting pattern:

11 inch width (1)
9 inch width (2)
7 inch width (2)
4 inch width (1)

That adds up to 47 inches, which allows for some kerfs or errors. If you cut the thin stuff first you may end up with the final strip being 12 inches wide, instead of 11 -- and that will work OK, too.

You'll use 5 strips for the pontoons, in this order 7", 9", 11", 9", 7". That gives you a pontoon that is 10 inches wide and up to 11 inches deep. The 4 inch wide piece is a stiffener or space which you can cut down to two pieces, wach 4 feet long, and then laminate into a 4 inch square part. Determin how far apart you want your pontoons and cut this to the appropriate length. Hold in place with a pin made from a dowel rod, and glass this into place if you want to join the pontoons permanently.

If you want to make a boat that can be taken apart, use the construction adhesive to glue a piece of 1x10 spruce, pine or fir, or a piece of 10 inch wide, 3/4 inch thick, plywood to the top of the pontoon plastic. If you go with the solid wood your material will be only 9 1/2 inches wide, but you can center this over the foam pontoon and round the edges of the foam to make it look nice.

Drill holes for, and insert, several stainless steel "t" nuts in this wood before you glue it to the pontoon plastic. Pack each "T" nut hole with a cotton ball and dribble a little brightly colored candle wax in to seal it. Now, after shaping the bottom of the pontoon to round it and remove the "stairstep" look, you can cover the entire pontoon and board with fiberglass cloth and epoxy resin. When the resin has hardened, you cna drill a small hole in those brightly colored areas under the glass to open up your "t" nuts. "Ream" them by hand with a pocket knife to cut back the overhanging glass and restore the correct diameter to the hole for the "T" nut. Now you can bolt any crosspieces to the strong top of your pontoons, and assemble your deck as you wish.

Simplest deck is a 4 x 8 foot piece of 3/4 inch plywood, stiffened with some chunks of 2x4 bolted to the bottom. If you want the front raised, there is no need to bend the plywood, just add on a second piece that is at an angle. This is a boxy pontoon job. No need to glorify it with fancy rounded curves

Hmm. while this has been discussed as a crabbing tool, or play toy, with a displacement somewhere over 500 pounds, it could be used alongside a dock, or in open water, as a platform for entry, & re-entry into a kayak. It would be low to the water so you could sit on the edge and slide from it into the kayak -- or out of the kayak -- making a convenient tool for teaching.

No reason you can't take another sheet of that foam and cut it into a box. It owuld make a dandy cooler for liquid refreshments, or if the box is big enough, a live well for those trophy-sized fish.

PGJ