In the ocean, density is determined by several factors, including the amount of pressure the water is under, how much salt is dissolved in the water, and the water's temperature. The more pressure water is under, the more it is compressed, and thus the higher its density becomes. The pressure in the ocean increases a lot as you dive downwards. The ocean is, on average, 4 km deep, and at those depths, pressures are very high. Density is also affected by how much salt is dissolved in the water. The salt content of sea water is called its salinity and the higher the water's salinity, the higher its density. The typical salinity of ocean water is 35 grams per liter, which is equivalent to ~7 teaspoons of table salt per 1 liter of water (or 2 teaspoons per cup of water). Last, the water's temperature influences its density. In general, the colder the water, the closer the molecules squeeze together, meaning the less space they take up and the higher the density.
Since temperature, salinity and pressure are different at different places throughout the world's ocean, the seawater's density is different in different places, too.

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