Don't know where GP is from, but "Süßwasser", which translates to sweet water is the German term for water from lakes and rivers in comparison to salty water from the sea. Might be the same in related languages.
Fun idea you got, but not exactly, in some european languages, "sweet water" refers to drinking water, as opposed to what would be "salt water", water from seas and oceans.
Acqua dolce in Italian is literally sweet water opposed to acqua salata, salt water. It's a common mistake to translate literally expressions and grammatical constructs from one's native language to a foreign language. I probably did it this reply without noticing it: it's long enough to contain some bug.
I would not say it's common: I'm a native (American) English speaker and I've never heard the word before! I would just guess it's some kind of flavored water before reading this thread.