There is the danger of accumulated fumes catching fire if there is a spark when an electrical device is switched on.
A huge explosion.
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.
You have a high risk of blowing up you boat.
There will certainly be vapour from the fuel present and lighting of any naked flame could cause an explosion of this vapour.
Gas fumes will stay in the bottom of the boat.
Ventilate the engine compartment, if enclosed.
check the tank
An important step when refueling a boat is ensuring electrical switches are off.
closing all hatches and doors while refueling
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.
An important step when refueling a boat is ensuring electrical switches are off.
Fuel vapours will linger in the cabin