I'm not sure if the 240 has one but that's the water pump button. Turns the fresh water pump on/off, for sink faucets, showers, etc.
That panel is common across models so it's possible to have a button that doesn't go to anything on your boat but it does on another. My 280 has a water pump and I have to press that button for it to power up. Once up to pressure though, it cuts out but will turn on automatically when the pressure drops if the button on the dash is left on.
Doug
