Glass fiber composites (GFRP) are common in civil engineering projects, but not in marine structures. One reason is that seawater effects degrade GFRP composites mechanical properties and interlaminar shear strength (ILSS). Here, influence of seawater environment is studied to determine the best composite materials for marine civil engineer applications, studying the influence of several factors in their mechanical properties. This is to determine safety factors to use in the design of structural calculations for marine applications. Glass/epoxy composites are the safest materials to use in marine civil structures as mechanical properties degradation becomes stabilized after moisture saturation level. UV and water cyclic analysis must be done to determine affection to transversal strength. Only vinylester GFRP has problems with biodegradation. GFRP fatigue performance is not influenced by seawater environment.