Hi Jeremy, the most common router bits for cutting a plywood kit are 6mm or 1/4" diameter straight bit.
For good marine plywood you can buy from Chesapeake Light Craft in Annapolis (
https://clcboats.com) or try World Panel Products in FL or NC (
https://www.worldpanel.com/). I recommend that you use okoume plywood. If you want to use meranti or mahogany plywood then at least use okoume for the bottom panels in the bow. Whatever you do, don't use fir plywood anywhere in your boat. It is unstable, warps out of shape and the surfaces crack badly.