Pour foam is generally utilized to expand within an existing cavity or form. You would have to build a form to get a basic shape for the pour foam anyways. I is likely not the strongest or most cost effective method. It can be done, but please resin coat the foam (or any foam/core/scrim) for that matter prior to glassing. We have seen many delamination issues from folks not resin coating cores prior to glassing.
1/2' Coosa is an expensive and heavy way to core a deck. 3/4' Nidacore or Divinicell may be a more cost effective and lighter option. Additionally, Coosa will wear out blades very quickly. Nidacore or Divinicell can be cut with a razor knife. Coosa will work, but will be quite labor intensive for little benefit. If you plan on adding a tower or T top later, I would make sure to have Coosa or equivalent high density core where the mounting pads will go.
