Does anyone know why does climate/water movement cause some parts of the ocean to be saltier than others? I'm stuck. Are there multiple ways?

4 answers

river runnoff (like the Mississippi, or Amazon) is fresh, so currents in the ocean which move that fresh water will be less salty.
What if it sits in a quiet cove in the hot sun and some of the water evaporates leaving salt behind? What if a river runs into your bay and mixes fresh water in with the salt? What if it rains a lot in a shallow bay?
The Baltic Sea is quite fresh, rivers run into it and connection with the North Sea is narrow.
Oh by the way if ice forms, salt remains behind below in the water which therefore is saltier.