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Is it possible to have planets with fresh water seas instead of salt water seas?.... And if so, how difficult would the circumstances be?
To answer this, you must know why Earth's oceans are salty. The salt in ocean water is a mix of the minerals sodium, chloride, sulfate, magnesium, calcium, potassium, bicarbonate and bromide. These are nowadays washed into the oceans by rivers, whose sediment is rich with these compounds gathered from their meandering courses through the land, which liberated these salts from the rocks they flow over. Ancient ocean bedrocks can also release salt, because those rocks were often from ancient volcanoes, and their contact with the water dissolves these compounds in the surrounding water, making it acidic. The acids dissolved minerals from lava, producing ions. More recently, ions from eroded rocks entered the ocean as the rivers drained into the sea. When these salts are deposited into the ocean by the action of rivers or otherwise, they have no-where to go and thus either settle on the seafloor or float in the water.
Overall salt density in the water depends on the evaporation rate of the ocean. Evaporation can be roughly equaled to a number of factors all combined, such as the size of the ocean, its temperature, the amounts of salt deposited in it and so forth. All oceans since their early inception on our planet have maintained a balance of water evaporation/condensation and salt depositing with these factors. Most global oceans nowadays have around 35 parts per thousandth of salt in their waters. In some places, overall evaporation can be too great for whatever reason, and all the water can disappear and only the deposited salt remains, creating a salt flat. With lakes, the water evaporation is almost non-existent unless they are seasonal, and not enough minerals are present in the sediments to altogether give the water any great salinity. In some lakes, there can exist a perfect combination of minerals and temperature that the waters can be salty - often quite so.
So, it would not be very hard for an exoplanet with the right surface conditions for the maintenance of a atmospheric water cycle to have low-salinity or non-saline seas. It would just depend on the evaporation-rate of the water and the amount of sediments that are deposited in whichever body of water by rivers or other forces (like volcanism). An example would be a planet orbiting a star with a lower metallicity then our sun, or maybe a planet whose surface consists of many, many shallow but large inland seas with little to no geological activity beneath them or much incoming sediments in their river deltas. That being all said, salt in the oceans may have played an important role in the development of life on Earth
- so such less-salty worlds might well be barren, at least of life as we know it.