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)