You're kidding right? I tell you what, how about you mix up a tank of 18% O₂ / 45% H / 37% N₂ pressurized to 230bar. Then open the tank valve next to an old outboard motor or a deckhand smoking a cigarette.
Sport diving with flammable mixes is something that only works in hacker fantasy land, not the real world.
Smoking would be bad because the fire would be amongst the tanks, but you'd have to open it almost on top of the motor to cause a fire that way, because the fire in contained, and the hot part (the exhaust) is under water. Plus the motor is not located where the people/supplies are.
So simply don't carry the tank over to the motor and open it there, and don't smoke. Even the most basic of safety practice would make this not a problem.
Do you not carry any fuel onboard? Hydrogen is safer than gasoline, yet people carry it without a second thought.
Nothing much would happen, the hydrogen would rapidly rise up and away.
You'd have a hard time intentionally causing a fire with a low level leak of hydrogen near a closed fire like an engine, never mind by accident.
A small amount of safety precaution (and diving is quite full of safety precautions, so I think they could handle it) is all you would need to make it safe.