Four different composite material systems are subjected to nine months of in situ exposure at a potential tidal energy site. These four systems are fiberglass/epoxy, carbon fiber/epoxy, fiberglass/vinylester, and carbon fiber/epoxy pre-preg system. The loss of shear modulus due to this exposure is observed and these values are compared to unexposed samples of these same material systems. The measured reduction in shear modulus of the four systems following nine months of in-situ exposure is 66%, 26%, 13% and 7% respectively. In addition, three panels of the fiberglass/vinylester system are exposed to accelerated testing in the laboratory and the change in shear modulus due to this exposure is also calculated and compared. These panels lose 38%, 33% and 33% following 30 days of exposure to artificial saltwater at 30˚C, 40˚ C and 50˚C respectively. It is recommended that tidal energy device developers interested in using composites should focus future developmental efforts on fiberglass/vinyl ester composites, due to their low cost and reasonable long-term durability, or on carbon fiber/epoxy pre-preg, due to superior long-term durability.