Make sure when you are selecting the marine caulk or sealant for your job that it is marine grade. This will ensure that it will keep moisture out and prevent damage like corrosion or rotting. Also, when selecting the right product, you will want to make sure you know what you are trying to bond together. Different materials work better with different types of sealants. For an all-purpose marine sealant great for use above the waterline, the 3M 5200 Polyurethane Fast Cure Adhesive is a great option. It bonds with almost all materials and works great for sealing holes and deck fittings.
For your fiberglass surfaces, Boatlife Life Seal Silicone-Polyurethane Sealant is fast curing and provides a water-tight seal, and is perfect for joints with movement. It can be used for both above and below the waterline applications, and is perfect for underwater seams. Boatlife’s Life Calk is a long-lasting marine caulk that can be sanded and painted and helps protect against harsh marine elements like oils, gasoline, and diesel fuel.