Ensure you don't have corroded connections and the boat has full power.
If the problem still happens disconnect the radar scanner cable from the unit and try. If the unit only shuts down when the scanner cable is connected then you have short in the scanner cable or scanner unit. It would need to be troubleshot by a tech but most commonly it is the scanner cable or the modulator board that causes a short. You might also open the scanner and inspect it for water or other signs of shorting.