How on earth do you measure the fuel level in a tank on a floating platform? Seeing as it's constantly sloshing around... Can't get my head round that.
Many ways. You can measure the capacitance of the remaining fuel but that still depends on the fuel height to some extent. You can ultrasound or radar the air space above the fuel in the tank to measure the empty volume. You can have three float senders measuring the height at three points in a triangle and send the average height to the gauge. You can know how much you put in exactly and use a flow meter on the outlet to count down against the starting volume. Or you can bond a window into the tank and look.
If you think that's hard, how would you make the fuel system work in a spacecraft?