Yes, but only slightly because water is highly incompressible. To take the example form wikipedia: Even at a depth of 4 km, the volume of water only decreases by 1.8%.

Yes. Pressure and temperature are the main factors influencing the speed of sound through water, which is very important when calibrating sonars as you need to know how quickly the sound will travel to know how far away a response is.

[https://www.arc.id.au/UWAcoustics.html](More details)

Yes. Some things I haven’t seen mentioned yet:

Although water can’t be compressed there’s a few other factors.

1. Density changes with temperature, and it’s a lot colder at the bottom then the top.

2. There are air bubbles in the water, all of it. Although atmospheric air mixing by waves is unlikely to make it all the way to the bottom, biology produces gases via several different methods and gas can be compressed, which reduces the space between water molecules and increases the density.

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).

https://en.wikipedia.org/wiki/Properties_of_water#Compressibility

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.