The readings are only approximate. The gauge is activated with the ignition switch. Rocking motion of the boat during normal operation will cause fluctuation of the fuel gauge. For a more accurate reading, make sure that the boat is level and at rest. When the boat is placed into initial operation, do not run the boat below a quarter of a tank until you have refueled several times and have a sense of how long you can operate the boat on the fuel available. Extending usage beyond the known capability may cause the boat to run out of fuel and strand you away from the shore.
Although it may be possible to see fuel in the bottom of the fuel tank, you still may not be able to operate the boat. The fuel pick-up system was designed to avoid introducing the water and debris that inevitably accumulate in the bottom of the tank. Rather than relying on visual inspection, you should pay attention to the fuel gauge.
Further, it is not recommended to allow the fuel to fall below one-quarter of a tank full at any time as it may result in damage to the fueling system.