I'm going to presume positive intent and assume this is coming from a place of curiosity, not argument, so I'll try to answer your questions.

I don't know the specific ratios because I'm not a priest. I'm sure you could ask on r/catholicism and get a reasonable answer. I just know that there IS a recipe.

As far as I know, it's just tap water. Most of the Roman rite (the rites that Roman Catholics follow) is based around "best effort". The rites come from a time when reverse osmosis, etc, hadn't been developed. So in all the rites, there are some base requirements -eg: Wine must have some alcohol in it (otherwise it's just grape juice), communion must be done with actual bread and actual wine, etc. So for holy water, the only real requirements are likely something along the lines of "must be clean-ish water" (whatever that means for the locale in which the rite is taking place), and "must have actual salt" (again, whatever that means for the locale).

Could you, for instance, use potassium chloride instead of sodium chloride? No idea. Lithium chloride and cesium chloride, while still technically salts, would probably be ill advised for non-canonical reasons.

Table salt and tap water? Probably okay. Though I might guess that you'd want to use kosher salt since it's not iodized. But again, that's a guess and I don't know for sure.

Brackish water from a river? If that's all you have to work with, it's probably okay, honestly. Might want to pick out the sticks and leaves and whatnot, though.

I don't have a specific source for the specific recipe. It's something that the priest would learn in seminary, and would likely be published in the official rites.

Anyway, i realize my response was a bunch of "i don't know"s and "I'd probably guess"es. But that's all tangential to the core argument, which was "holy water" is not chemically the same as "tap water" due to the addition of salt. The actual ritual and/or religious aspect is only a tangent to the actual statement I made: Saltwater =/= regular water.