Why they should use seawater? Fresh water is not in shortage in this region - yearly precipitation significantly exceeds evaporation.
Finland has vast stores of fresh water and annual precipitation has nothing to do with it.
Or is it the the sun and rain that created the lakes?
Thanks to the last ice age and retreating glaciers...
The fact that they are still full of water is due to them being replenished by precipitation more than they lose water. If they would lose more water then what flows into them we would be speaking about the "land of many bone dry holes" in the famous Finnish desert.