How can I insulate my porch floor?
Indoor / outdoor carpet?

Maybe you can insulate by carpeting on top of the floor you have.  There are lots of options.  This will leave air movement under the porch as it is which I think is smart given flood risk.  The indoor / outdoor carpet can be hung up to dry if the river floods, and replaced if it wears out or tastes change.    

Anticritter mesh is a good idea, though.