The ocean is maintaining a constant salinity because:

In mid-ocean ridge areas, ocean water is cycled through the crust because of the high heat flow in the area. Water in the rock is heated over the magmas, rising up and drawing in cold water from the sides. This process is occurring all over the world, and is the primary means for maintaining ocean salinity in a relatively constant range over the long term, as elements in the sea water when it is heated react with the rocks. There is so much water flow, cumulatively over the entire world, that this process basically buffers ocean chemistry.

Obviously large increases or decreases in salinity can and do occur in isolated basins that do not have free exchange with the open oceans.

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