To achieve in situ underwater chemical sensing, a commercially available Raman spectroscopy system was adapted for underwater operation. Using off-the-shelf laser unit, optical probe, spectrometer, and power/communications module, the system was integrated into a custom designed pressure housing. This instrument was deployed on the FITS (Fiber-optical Instrumentation Towed System) developed by the National Sun Yat-sen University for experiments. Measurements of Raman spectra were successfully performed with the instrument at 500 m depth. Power supply, instrument control, and data collection were provided through the FITS, which was connected to the research vessel with a cable containing copper conductors and fiber optic cables. Raman signal of sea water was successfully obtained from the measurements. Evidence of possible detection of methane was also recorded in the measurements.