I've double checked this so I don't get somebody hurt. Even if the welder made the water boil, water boils at 100C. It does not exceed 100C until you have 100% steam.....ie, for the time you are in there making a small TIG bead it will certainly stay at 100C.

The minimum flash point, or min ignition temp) for gasoline is 260C

.......Its a little unconventional, and for a few dollars alot of welders will not see the supposed risk as worthwhile, etc, But I think this could be done.