Water near the poles often have higher salinity because Cold polar air cools the water and lowers its temperature, increasing its salinity. Fresh water freezes out of seawater to become sea ice, which also increases the salinity of the remaining water.
They have the same density because a material, no matter how much of it there is, will always be a certain density. A 40g ball of iron has the same density as a 1g ball of iron.
