5200 is not an appropriate "sealant" in most applications. It is very, very difficult to remove without damage.
In most applications involving fibreglass, I use Sikaflex 291 or the Sikaflex UV equivalent. In the case of the Beckson plastic port, I used what the manufacturer specifically recommended, otherwise I would not use silicone on the boat.
A bedding (sealant) material that I have not used, but have heard great things about, is Butyl tape. Rod Collins, a member here, sell a good quality product, and not all are the same. marinehowto.com