I'm a newbie DM, but if a player wanted to "purify" a person, essentially killing them, I would tell the player, ok then, if you want to do this, then other low level clerics would have this ability as well. Do you want the party (even at high level) to potentially be killed off by low level clerics? i.e. to keep it consistent, anything a player wants to do could potentially be done to them also, so be careful what you wish for.
As for the original question, it's pretty obvious, to me anyhow, that it would apply to salt water as well.
As for the original question, it's pretty obvious, to me anyhow, that it would apply to salt water as well.