I use parafin wax to occasionally reseal my wood canteen and I know others with leather canteens who do the same. Not sure how well either wax or brewers pich would work on a metal canteen since it may not adhere to the smooth surface. Never know until you give it a try.I think I'll get some brewers pitch and try to fix my canteen that way. It seeps and there are some rust holes in it.
Paraffin wax has been used for years when sealing home canned jams & jellies and I've never heard of anyone who came to any harm from it.I am positive your family Dr would tell you to NOT use any petrolium base substance to repair any type of food / water container.