Thermoplastic matrix carbon fibre composites offer considerable potential for underwater applications. There are various material options but there are questions concerning the compression behaviour and water sensitivity of the less expensive polymers (polyamides) for these applications. The aim of this study is to assess whether thick carbon/polyamide cylinders produced by tape placement could provide a low cost solution for underwater pressure vessels, by examining these two aspects. The influence of sea water immersion was examined first, in order to propose a diffusion model so that the water profile could be determined. Compression properties were then measured for dry and wet specimens. Before aging, values were comparable to carbon/epoxy, around 1400 MPa, but these were reduced to around 600 MPa in fully saturated specimens. Pressure vessel implosion tests on preliminary tube samples indicated an implosion pressures around 200 bar for 10mm thick 120mm diameter carbon/polyamide cylinders, but this should be increased by optimized manufacturing conditions. For deep sea applications alternative carbon/PEEK materials have been shown to provide the required implosion resistance.