The composite materials used at sea are today nearly all based on thermoset resins (polyester, epoxy). However, there is an increasing number of thermoplastic matrix polymers available on the market (PP, PA, PPS, PEEK…), which offer possibilities for forming by local heating, attractive mechanical properties and the potential for end of life recycling. The aim of this study was to design, manufacture and test thermoplastic composite pressure vessels for 4500 meter depth, in order to establish a technical, economic and ecological assessment of the use of these materials to replace traditional composites underwater. First, finite element calculations have been carried out to optimize the stacking sequence with respect to the external pressure and buckling resistance. Thick thermoplastic cylinders were then manufactured and tested until implosion, their behaviour showed a good agreement with calculations. Overall, the results show that it is possible to use Carbon/Polyamide 6 (C/PA6) thermoplastic composite cylinders for deep sea applications, as implosion pressures higher than 600 bar (6000 meter depth) were achieved.