I use the Walmart Camping Pad (blue) material to pad thigh braces that are out of wood . They are Ethafoam - basiclly a closed cell cousin - and will not absorb water. Those long tubes that children use in the pool are Ethafoam. Ethafoam does not carve well. You can cut it, but its tough to shape. Gluing up a blank with multiple layers will be impossible to carve. I would bite the bullet and buy the grey closed cell that kayak shops sell. It's expensive but carves well. There's not a lot of other choices.

Newfound woodworks, Chesapeake Lightcraft and others carry it. (you need wider than 1 ft. to make a seat.) You want 3" thick. As someone else mentioned, call various kayak shops. I think Ocean Kayak sells pre-carved ones - but a bit pricy.