The "correct" way to screw or bolt to a cored fiberglass surface is to remove the top layer of fiberglass, with the appropriate size holesaw (depends on expected load), then remove the core material and either epoxy in a phenolic backer or second best fill the void with epoxy/milled fiber mixed to about peanut butter consistancy. You can then replace the top plug, epoxy it in place, and screw or bolt to this much stronger area. Even better would be to glass in an entire fiberglass pedestall mounting surface with built in stainless t-nuts. The problem with just screwing directly to the surface is that it will eventually break away from the core, allowing water intrusion and the resulting failure of the floor. I'm sure this is not what you want to hear or what the salesman told you, but it is the correct way to do it. Probably one of the things that has caused damage to more boats than anything else is improper attatchment of fittings to cored areas. That, and improper bedding.