The condition that would most likely cause a decrease in the salinity of ocean water is a river flowing into the sea. This would make the salinity decrease, because rivers a fresh water and have no salt in them. Since ocean water is made up of salt, water with no salt mixing with water with a lot of salt will cause a decrease in the salinity. I know this answer, because I took a quiz on this, and the correct answer was a river flowing into the sea.