A study of thermoplastic matrix composites has been performed to investigate their use in underwater applications such as oceanography, submarine, and sub-sea offshore structures. This article first presents six candidate materials. Results from simple mechanical and seawater aging screening tests on flat specimens are then described. Glass/epoxy and carbon/epoxy composites are used as reference materials. Two materials emerged from this process, glass/PEI and carbon/PEEK. Cylinders of both were manufactured and subjected to hydrostatic pressure tests, and results are compared to those for glass/epoxy and carbon/epoxy cylinders of similar geometry. The carbon/PEEK material appeared most promising. It resisted pressures in excess of 90 MPa and was retained for damage tolerance assessment studies. Drop weight impact damage zones were smaller in carbon/PEEK than carbon/epoxy for the same impact energies but the loss in residual collapse strength was more rapid in the thermoplastic composite. This was attributed to a change in failure mode, impact damage initiated a local buckling failure.