"Freshwater" as opposed to salty water

Solution 1:

The NOAD identifies the etymology of fresh:

ORIGIN Old English fersc [not salt, fit for drinking,] superseded in Middle English by forms from Old French freis, fresche; both ultimately of Germanic origin and related to Dutch vers and German frisch.

Sweetwater, I would assume, is at this point (don't know about originally; cf. @BarrieEngland's answer) the antonym of saltwater, and therefore synonymous with freshwater.

Solution 2:

The OED’s earliest citation for ‘fresh water’ is 1528, in the phrase ‘the best freshe water fyshe’. ‘Sweet water’ appears around the middle of the sixteenth century, in the sense of ‘a sweet-smelling liquid preparation’. It is first used to mean fresh water in 1608.