you would need to reroute it directly to battery and not go through any switches to make it an automatic pump with the float switch.
If you just add the float switch, then as soon as you turn power off, there is no power to the float switch. If you were to forget the drain plug and launch the boat and not have the power turned on, the boat will fill with water anyways. If it is directly wired to the battery, then the float switch will have power and sense water, thereby turning on the pump.