-- Last Updated: Dec-29-12 4:24 PM EST --
As Jack said, you want to use epoxy. Auto body filler is OK for just fairing in small defects like chips in the gelcoat, but it really doesn't have any structural strength.
It sounds as if your boat has a molded keel (the ridge you described). Many less expensive fiberglass boats had either a shallow, molded keel (projecting down into the water) or keelson (projecting up into the hull). These were typically added not to serve as a functional keel but to add stiffness and rigidity to an otherwise somewhat floppy hull bottom.
My guess is that the boat is probably not worth investing a lot of money in, but fortunately it won't take a lot of money to fix it. I would suggest you buy this kit:
This will probably give you enough epoxy to fill in the holes and fix the chunk missing out of the keel. To fill holes, it is best to use the colloidal silica powder (the white powder included in the kit) to thicken the epoxy. The fairing powder (the brownish stuff) is used to thicken the epoxy a bit but is easily sandable so that you can get a nice, smooth and fair surface before painting or laying on fiberglass cloth. The kit also includes some E 'glass cloth.
Before applying the epoxy you want to sand the area of repair until you see fiberglass cloth fibers, then clean it well with something like isopropyl alcohol. After completing the repair, wash the epoxied areas with soap and water and rinse well to remove the amine blush. Then just spray paint with a color that approximates that of your hull.
Hardshell Kayak Sail Rigs