Fresh (non-salt) water is also called sweet in English. Surprisingly, the word sweet is also used in some Slavic languages.
To be precise:
- in Polish,
- in Czech,
- in Croatian (as well as in Serbian and Bosnian, may be with different spelling),
Also, there are some Slavic languages where literally
питьевая means "пресная" (it worth to mention that the same we can tell about above-mentioned languages, it's just that this expression are not 100% interchangeable)
For example, in Macedonian,
Though in Bulgarian, which is close relative of Macedonian, it is said
прясна вода. In Ukrainian one should say
прісна вода, in Belorussian,
прэсная вада. But the two last examples are not that surprising, since both Ukrainian and Belorussian belong to the same subbranch of Slavic languages that Russian belongs to as well.
The question is: Has it ever existed expression "cладкая вода" in Russian, with that very meaning. If yes, when and how it loose battle to "пресная вода"?
UPD: here is an excerpt from Church Slavonic dictionary:
сладкїй - cладкий; весьма приятный; добрый, кроткий.
This does not give any hint.