Fiberglass is a lousy material for holding screws. The glass fibers will break and the resin isn't very good at holding screws. Your best option is to fill with thickened epoxy and then re-drill and use the screws again. If you can through bolt instead, you'll probably have much better results in the long run doing that instead of using screws.
If the fittings in question have any kind of load on them, you're much better off through bolting, rather than screwing them down. The epoxy filling is also good for helping seal the core, if you have a cored deck.