On the small deck pieces I have some advice, make a supported joint by using a scrap piece of ply and make a support, screw and glue it on the underside of the larger section protruding out a few inches so the small pieces don't have that edge just hanging there with nothing under them at the joint. You should glue and screw from the top through the small piece then after laying it in the larger section. Unless you can do it out of the boat and get the large section in. Guess it would be similar to a scarf joint type set up.
Psst, in a boat a floor is called decking :lol: