How does water get inside the fiberglass laminates?

There are several ways for water incursion to happen.  Below the waterline over time the
pressure exerted by the water will push the water through gel coat into voids in the laminate
formed during the construction/layup process.  Gel coat is not totally waterproof and some
types of resins are more permeable than others.  Vinylester and epoxy resins are most
resistant.  Be careful to use the right products for hull repair.  Above the water line, damage to
the gel coat will admit water.  Poorly sealed or loosely mounted hardware will allow water to
penetrate the protective gel coat skin into the core material.  In northern regions you can
imagine what freezing and thawing of this moisture will due to the laminate structure.  Water
incursion can be prevented by careful manufacture of the fiberglass laminate.  Modern
methods such as SCRIMP molding all but eliminate voids in the laminate.  Correctly mounting
seacocks is vital to prevent seawater seeping around them into the hull structure.  All
deck-mounted hardware should be painstakingly sealed, and resealed every 7-10 years.