Here's why:

* Salinity: Ocean water contains dissolved salts (mainly sodium chloride). These salts add mass to the water without significantly changing its volume, increasing its density.

* Temperature: Colder water is denser than warmer water. While ocean water is generally colder than freshwater, the salinity effect is stronger.

Therefore, even though ocean water is typically colder than freshwater, the dissolved salts make it significantly denser.