I was asked about the best way to repair existing carpet snap holes in the non-skid of a Sea Ray Sundancer.
Epoxy putty is the best choice for filling screw holes in fiberglass. You can mix your own, but for your use, a pre-mixed epoxy paste such as white Marine-Tex will be easier and result in a less visible repair. If your non-skid is not white, you can tint the paste with a coloring agent.
The process is easy. Use a countersink bit to chamfer the top of each hole. This cleans up the contact area and makes the repair stronger by giving the epoxy a larger and more horizontal surface to adhere to. Without access to the bottom of the hole to seal it with tape, you may want to fill the holes in two steps. First put just enough epoxy putty into the hole to seal the bottom. Where holes are large, the putty can tend to drain through. In this case, use a bit of dowel to create a plug, or stiffen the paste with sawdust, talc, silica, or even a bit of wadded tissue. You're just trying to close the bottom of the hole.
When your plug has set, or if you don't need a plug, fill the hole completely with your epoxy putty, making it level at the top with the surrounding surface. Epoxy does not shrink during cure. As this is a textured surface, you can match the texture by stippling the putty with a finger, a cloth, or some other tool that gives you the desired effect. When the epoxy has cured fully, you'll have a permanent repair.