Salt? I don't know if there are any in finland tho.
Does finnish differentiate between fresh and sea water? If it's near the coast I imagine it could have been named thus to differentiate it from a nearby gulf?
"Vesi" means generically any kind of water and there are compound words for seawater and freshwater just like in English. There are several Vesijärvis in Finland, and now that I googled it, it's conjectured that the word vesi in the name does not refer to the water in the lake but rather derives from an archaic word meaning "outlet river".