Some parts of the ocean have lots of rain. The freshwater added at the surface dilutes the seawater, reduces the salinity and so makes the seawater less dense. Seawater can also be less saline near land, where rivers add freshwater.
The ocean around Antarctica has a low salinity of just below 34ppt, and around the Arctic it is down to 30ppt in places. Thawing icebergs add freshwater – icebergs that have broken off ice sheets formed over land do not contain salt, and the freezing of seawater into ice floes removes more salt.
When water freezes, a rigid open lattice of hydrogen-bonded molecules is formed.
The Baltic Sea, almost enclosed by northern Europe and Scandinavia, has a very low salinity of about 10ppt. This is mainly due to the huge amount of freshwater added from hundreds of rivers.