Figure 1. The salinities of Earth's Oceans measured in ppt
The density of water is determined by its temperature and salt content. Water density increases as its salinity increases, and it increases as its temperature gets colder. So, warm, less salty water floats on top of (is less dense) than cold, more salty water. These differences in density of seawater have profound effects on ocean circulation and the transfer of important elements like nutrients.