Originally Posted by BA_Colt
^^^what BA said
It can be repaired from the top only. I fix boats for a living and unless the crack goes all the way through, a top repair only will do.
IMO it is best to grind away the damage and to either side of said crack at least 1" . Fiber glass it just a scosh high.
When cured block it out straight (sand) then prep for paint or Gelcoat. Ya it's sucks to do, but could save you a replacement cost.
Tips for grinding and sanding: run a shop vac to keep down the itch!!
Good judgment comes from experience, and a lot of that comes from bad judgment.