There are many products advertised to the boat (and home and auto) owner for fiberglass repair. Unfortunately far too many products simply “stick” to fiberglass. By using the word “stick” I mean, a product that only adheres to the surface of fiberglass. An example would be glue or caulking. When applied to a crack or hole, it is surface attached. It does not become a part of the fiberglass. It lacks equivalent strength and flexibility. The majority of these products come in a toothpaste size tube and are available in hardware stores. They are primarily designed for household repairs…not boat repair.
Many glues and caulk are water soluble. That means they will dissolve when submerged in water or soften in a moist atmosphere. If not water soluble, they will degrade in strength and appearance if not painted or coated with a protective film.

Many home handypersons make the mistake of using epoxy repair/filler products improperly. These generally are two part compounds you mix together. They will stick to fiberglass and work well to repair upper deck screw holes and small surface chips. However, they do require painting if a chip is visible. They are not suitable for hull cracks below the waterline because they are not of equivalent flexibility to fiberglass, regardless how strong they are advertised to be.
Remember: Improper upper deck repairs may cause your fish finder or drink holder to come loose. A failure of a repair to the bottom...could result in sinking the boat.