I never let the gas guage go below 1/2. The first year I had the boat I went out after supper for a little ride. I NOTED the gas guage said I had just a hair over 1/2 tank. That would be half of 40 gallons. No Problem. 30 minutes later I was out of gas. Bummer at night.
Boat gas guages are notoriously inacurate for two reasons: Firstly, the gas tanks are iregularly shaped to fit into confined irregular shaped spaces. So if the guage says you have 1/2 tank..that does NOT mean you have half of the tanks total capacity left.
The second reason is that the electro-mechanical float sensor mechanism in the tank takes a terrible beating, bouncing around in there every time you fly over and into a wave.
Experience is a good teacher..it makes you remember.
Nb