Don't use polystyrene. It has no strength. A foam sandwich boat with polystyrene core will act just like two separate skins - very flexible.
Polystyrene is insulation.

: Is this your first time using foam? Is this a foam plug for a
: fiberglass mold? Or do you use the foam boat as a mold? I'm
: considering making a slalom boat using a similar method. I love
: expanded polystyrene because it is easy to obtain. I'd like to
: see more of this build!