When water evaporates it leaves salt behind causing water in warmer areas that have more evaporation to have saltier water.
The Mediterranean Sea (from observation via thermal images from space) is one of the saltiest seas, also the Red Sea (to a lesser extent). Both the Dead Sea and the Great Salt Lake are so salty that most people find it easy to float. Lake Assal is considered the saltiest body of water outside of Antarctica.
The Northern Pacific is at the other end of the spectrum, containing relatively lower ocean salinity.
Water evaporation at phenomenal rate in warmer places causes accumulation of salt in respective area. Add to the fact that the warmer the water the higher the maximum saturation capacity and you have very salty seas. These are the main two factors of water salinity.
There are areas in the world where fresh water mixes with salt water, thus reducing the salinity. These areas include: 1) The mouths of rivers; 2) The melting ice pack, such as in the Arctic and Antarctic; and 3) Areas around Greenland, where the glaciers 'calve' off, making icebergs. This list is not all inclusive.