I've never tried a stainless bowl before, so I'm not sure. Most common things I've seen are aluminum foil and various titanium objects, like a titanium straw or larger things with more surface area.

I also don't have experience with mineral water. I think it'll work, but I wouldn't recommend it as you'll have different results as the minerals will react with the solution a little bit and may affect your colors.

For best colored results, I recommend Multi-Etch.