Here is a quick setup I have seen that is almost foolproof and will do the job. It only works while running however. Get a Y-valve, 2 scoop grates with directional fins and a waste or water tank that will fit. Position one grate facing forward and one facing rearward. Run hoses to both sides of the Y and then one out to the tank(s). Position the valve near the helm. When the forward facing grate is opened under way the tank will fill(tank must be vented). When the rearward facing grate is opened the tank will empty. A cutoff valve might be nice to hold the tank half full running or full when the boat is pulled from the water. Electric solenoids can also be used, but that is more complicated.