From the DMI web site
The S200DL will show MSD when the transducer is not in the water, in water under 2.5 feet or in water over 190 feet, this is normal since the instrument can not read in these situations. If it happens at normal depths this could be a fault in the instrument or the transducer.
The MSD18.8 reading is definitely in the instrument. This reading means that all the segments of the LCD are displayed at once. Just replacing the LCD may not be enough since the circuit that drives the LCD may have failed causing the LCD to fail. Replacing the LCD without testing the drivers could result in the same failure next season. By sending in your instrument for service we can check the LCD drivers, plus align and calibrate the transmit and receiver gain also replace any corroded hardware making your depth sounder as good as it was when it left the factory.
Your better off trowing it in the trash and updating.