Water is not very compressible; even at the bottom of the ocean a kilogram of plain water still takes up 0.982 liters of volume (compared to 1 liter at the surface).
However, actual ocean water is saline (salty), and the salinity of the ocean varies widely. Dissolved salt makes ocean water more dense than pure water; and the more salt there is, the denser it is. That's why it's easier to float in the ocean than in a freshwater lake.
Typically, the water at the bottom of the ocean is slightly less salty than the water at the surface. This is because evaporation happens at the surface.