The density of water at the bottom of the ocean is actually quite similar to the density on the surface; it differs by only a few percent. Gases compress proportionally to pressure, but liquids act more similar to solids and compress very little even under enormous pressures.
The oceans are only about 3.5% salt by weight, so that doesn't make a huge difference, either.
The oceans are only about 3.5% salt by weight, so that doesn't make a huge difference, either.