The boat could blow up. Gasoline fumes are very explosive. That is why boats are required by law to have ventilation for the engine room. If there are fumes and there is a spark from something electrical then the boat could explode. Always ventilate after fueling for at least 4 minutes, and make sure that electrical equipment on the boat is UL listed as ignition protected. That means it won't ignite any gas fumes. Always check for fumes with your nose before starting the engine.
Gas fumes will stay in the bottom of the boat.
Unusually nothing happens. BUT a build-up of fumes is possible under the right conditions. That along with an ignition source (engine starter motor etc..) can cause a catastrophic explosion. Use the blower while fueling and before starts IAW your owners manual.
You have a high risk of blowing up you boat.
Ventilate the engine compartment, if enclosed.
You need to ventilate your boat after refueling to disperse any build-up of flammable fumes - especially petrol (diesel is less dangerous). Any petrol fumes sinking into the bilges could be ignited by a spark from an electrical switch, or when smoking.
There is the danger of accumulated fumes catching fire if there is a spark when an electrical device is switched on.
There will certainly be vapour from the fuel present and lighting of any naked flame could cause an explosion of this vapour.
extinguisher
A huge explosion.
closing all hatches and doors while refueling
Turn off the engine and any devices that could create ignition. Close all doors and compartments so as to keep fuel fumes from getting trapped in these places.
Run the ventilation blower for a minute after fueling.