Surfboards are made with glass over various types of shaped foam cores. It seems OK for their purposes but in boatbuilding it falls short. Paddle boats are expected to last a lot longer than surfboards. They are hollow inside so they have carrying capacity. And being powered by human muscle a paddleboat has to have a nice slippery shape.
Trying to get all this with a quicky styrofoam plug has never proved to be practical.
The big problem with any glass hull is it has to be spot on when it comes off the mold. If you produce a hull with a cobby exterior finish and you take a grinder to it to knock off the high spots you most likely will cut into the glass laminations to the point of weakening the entire hull. Glass hulls are not particularly light because of the large resin content. So, if you have low areas and you fill them the hull weight goes up very fast.
If you read "Building Your Kevlar Canoe" (it's really about building glass canoes with a layer of Kevlar) you will notice that the mold that is built takes as much or more time to construct than the boat. If the mold is flawed the boat will be too. The guy in the book went over his mold so many times you get tired just reading about it.
The boat constructed in the book comes off the mold with a couple of extra layers of epoxy to bury the glass. When the hull is ready for sanding these last thin layers of epoxy are all you have to rub out any imperfections. If you sand aggressively you will cut into the glass and harm the hull. There is no way to fix it that will not mar the appearence of the boat.
Also, building glass hulls is very labor intensive and getting quality results takes a lot of experience. That's why it has faded in popularity over the years.

: What is the feasability of making forms from the 2" thick 8'x4'
: styrofoam sheets from home depo? Then you could add forms between the
: forms to fill in all gaps, sand smooth and glass it over...then do the
: cockpit.

: Sounds like a fast way to make a boat?