Doesnt seem to fluctuate at all. I use the same location each time i get more water. I always get the water on a rising tide even the the location i get the water is always covered in water (tide doesnt go out this far).
I get my water from the atlantic ocean, though this part of the atlantic ocean is the gulf stream and this stream hits our part of the coast head on. Maybe this helps to keep water conditions at a high quality.
Dont increase it too fast. Perhaps a little salt extra every couple of days. this will gradually increase the salt levels t o a standard you are more happy with.