This article is the third of three papers describing a study of the monitoring of filament wound composite cylinders for underwater applications. Part I described the technological issues and the development of specimens instrumented with embedded gratings and thermocouples, with the aim of monitoring the temperature and strain changes during the cylinder manufacturing presented in Part II. Residual strains are not negligible, over 1,000 axial micro-strain at the end of the curing cycle. Part III describes the response of these cylinders to hydrostatic pressure loading. The same embedded fiber optical Bragg gratings (FBGs) used for parts I and II of the study are here used as strain gauges. Their response is compared to that of classical resistive strain gages bonded to the inner surface of the tube. Results from these initial tests demonstrate the embedded FBG sensor's capability to monitor structural health of an underwater structure from fabrication throughout its service life. Embedded instrumentation records strains during pressure cycles up to final failure, without affecting the cylinder response.