Hey Tyler,

Let me be the devil's advocate regarding plywood. I've seen competitions where boats are made out of cardboard, but that doesn't make it a great idea. Marine grade plywood is more than simply being solid core, the glue is spread edged to edge, where exterior grade plywood may not be. Exterior grade ply glue is designed to withstand dampness, where marine grade is meant for immersion. If you are looking to build something that will be used occasionally in calm waters, probably not necessary to go for marine grade.

However, children are much harder on boats as they tend not to baby the bottom or worry ramming into things so the opportunity for moisture penetrating into the wood is high. If you intend to take your son on some longer trips or go camping off your kayaks, go for the real thing. I believe marine grade ply exists for more than marketing purposes; when you are on the water your life depends on your boat. I'm an advocate of using the best materials you can when it comes to this kind of thing.

Just my two cents.

Eric