I would go check out the Hobie forum on thier website. Lots of guys on there with lots of experience doing repairs.

My suggestion is to notuse Marine filler but use an epoxy (like West Systems) with a silica filler to make an almost toothpaste consistancy mixture to buld up the keels close to the finished shape. Let ti scure about halfway then add the final layer to build it out the the final shape. Mix this layer with micro ballon filler. This will allow any paint or surface treatment to adhere better.