Sounds like
dinghy fuel to me.
I've had gasoline on several cruising cats. I've also working in and designed refineries. Fortunately, they are able to hold gasoline on the bridge
deck, in fuel holds that are gas tight, ventilated out the bottom, and contain no
electrical equipment. On a
boat, as a minimum, I'd want to make certain the bulkhead was not just liquid tight but also gas tight. That means no penetrations clear to the
deck. Second, the compartment must be ventilated from the bottom and top. Finally, no
electrical connections, machinery, or
lighting, or even the backs of deck lights. Nothing other then the
tanks sending unit.
According to the National Electrical code for shore installations, as soon as you put a gasoline tank in a closed space, the entire space, in this case including the
hull up to the deck, is a classified space that requires explosion proof
equipment. For example, if you put a 50 gallon tank in a dike behind your shop, the space inside the dike, up to the lip of the dike and within 10 feet of the tank, is classified. And of course, if the
tanks is in a building, the vent must go through the roof. All in all, this is a tough standard to meet in a boat.
And I've seen the explosions, in refineries and one at the fuel
dock.
It can be done, but you need to take it very, very seriously. Buy a copy of the relevant codes, remembering that these codes are not written by the
government, they are written by the industries to reduce their losses. I've served on code committees.
A smaller tank, that could be above the
water line in dedicated sub compartment, like a
propane locker, is a better bet. Think
propane locker; they have much in common.