Biochemical indices are important measurands in marine environmental and ecological monitoring. Traditional monitoring methods of biochemical indices usually use chemical reagents and the operation is also time-consumed. Moreover, after a long measurement time, the results cannot accurately reflect the real status of seawater. In this work, four in-situ sensors are investigated based on the optical sensing principles, i.e. the chlorophyll a, chemical oxygen demand (COD), biochemical oxygen demand (BOD), and the underwater plankton imager. The development of the instrument prototypes and buoy-based in-situ trial experiments have been completed and described. During the sea trial, the environment-friendly antifouling paint and mechanical wipers have successfully overcome the biological fouling issues to the sensors, which have been kept operation underwater for more than six months. From the trial experiments, we have collected a large amount of real seawater data and maintenance experience of the equipment. The buoy trial results also reflected the true conditions of the seawater.