Drilling Into Fiberglass No Nos
Drilling holes through cored decking without taking the proper precautions is asking for trouble. Leaky lifeline-stanchion mounting bolts, (most likely due to failed caulking) have allowed water to enter the cored decking beneath. The mounting nuts are drawn so tightly they're crushing the panel, no doubt an attempt by the owner to not only keep the stanchion from wiggling, but also to try and stop the leak (note the inadequately sized washers and lack of a backing plate).
No backing plate, nuts tightened too far, and incorrectly sized washers are all leading to a leaky lifeline stanchion, right. The photo on the left shows how it should be done.
In a perfect world, your boat's manufacturer has anticipated where all deck penetrations are necessary and has "de-cored" these areas by reverting to solid fiberglass, allowing you to mount hardware without drilling into the core. But in the case of new installations, your chances are slim to none that any of these areas will coincide with whatever aftermarket doodad you want to mount, meaning you're going to have to do it the hard way. Anytime you screw or drill through a cored panel, the first rule is properly sealing the core against moisture entry.