Boats are a bit different to cars, but the fuel side is the same job. The tank might be built into the hull, tucked under the decking or sitting on the trailer, and the fill point is not always where you would expect. None of that is a problem, we work around awkward tank setups every season and carry the gear to draw the fuel out cleanly.
The common ones we see: someone tops up at the servo on the way to the ramp and grabs the wrong pump, a two stroke outboard gets filled with straight unleaded and no oil, or petrol goes into a diesel inboard on a cruiser or yacht. Different boats, same fix, get the wrong fuel out before the motor runs on it.
The one rule that matters most is do not run the motor. Same as a car, the fuel sitting in the tank does no harm, it is running the engine that circulates the wrong fuel through the parts it can damage. A two stroke run on unleaded with no oil, or a diesel run on petrol, is where the real bills start.
We come to the ramp, the marina, your driveway or the storage yard, drain the wrong fuel, flush the lines and refuel with the right fuel so you can get on with your day. It is the same drain and flush we do for petrol in a diesel and diesel in a petrol on the road, just at the water's edge.