The water at the bottom of the ocean is compressed by the weight of the water above it all the way to the surface, and is more dense than the water at the surface.
A consequence of compressing a fluid is that the viscosity, that is the resistance of the fluid to flow, also increases as the density increases. This is because the atoms are forced closer together, and thus cannot slip by each other as easily as they can when the fluid is at atmospheric pressure.
Skipy
Aircraft engineer, so I do understand hydraulics!