welcome
it would realy help if you add a few details like the size of the boat and the motor. If its a 15 foot jon boat then it should be full. If its a 31 foot offshore fishing boat with twin engines then $50 is a splash in the tank
thats a 30 gallon tank so it looks like the gas gauge is off as long as the boat is level and not tilted on a trailer. (not all gas gauges are set to the bottom as its better to read empty when it still has a few gallons left). If this was the first fill up i would surgest another $50 but if the boat had some gas already then i would call the place you puchased it as it might be under warranty