This article presents a study of the erosion resistance of coated and uncoated polymer matrix composites for tidal turbine conditions. It focuses on the development of comparative erosive wear mode and mechanism maps for such materials. In our earlier work, testing of glass-fiber-reinforced polymer composites for tribological applications in marine simulated conditions, several erosion-related issues were highlighted. The combined effects of the NaCl solution and sand dramatically enhanced the erosive wear of the uncoated specimens. In order to address those issues, an erosion-resistant polymeric coating was applied to the composite and tested in marine simulated conditions with an extended range of sand particle size. The test results of the uncoated and coated composite have been compared in this research by erosive wear mode and mechanism maps techniques. These maps reveal that the coating has enhanced the erosion resistance. These findings provide significant progress toward materials selection approaches to manufacture of tidal turbine blades.