What are three ways that the density of ocean water can increase?

Three of the most obvious ways the density of an ocean (can be applied to any body of water) are ->

temperature (colder water is denser down to 4 degrees c, but anything between 4 and 0 degrees c is lighter and floats to the top (that's why the top of water freezes before the bottom :) ) )

solute percentage ( in oceans a main solute is salt - we put more salt into the ocean and it gets denser, we filter out the salt and the water gets less dense)

population & pollution ( fish or soda cans don't directly change water density, but the waste products and disolved rust make it denser)

It is the second answer