what will prevent the ignition of gasoline vapors if an inboard ignition backfires?

Gasoline engines installed in a vessel after April 25, 1940, except outboard motors, must be equipped with an acceptable means of backfire flame control. The device must be suitably attached to the air intake with a flame tight connection and is required to be Coast Guard approved or comply with SAE J-1928 or UL 1111 standards and marked accordingly.

bad gas or bad plugs or bad wires,dist cap,timing off tune the bitch up

if your boat has a blower it will take care of that and you should always have fire extinguishers and other saftey equip.

All gasoline inboard marine engines are equipped with flame arrestors on the air intake to prevent explosions if the motor backfires.