I've had better luck not drilling a pilot hole before putting in a screw. I usually just use my power driver and screw them directly into the hull...snug them tight and stop. If you coat the screw with Rule before you screw them in, you have a water tight seal and solid hold all in one step.
If you do decide to drill first, go really slow. The drill bits have a tendency to grab the plastic and feed the bit in too fast resulting in a deeper than needed hole.
Good luck.