Replacing pool pump/plumbing etc. that is below the level of the pool
Use gravity, all you need is a garden hose! If the pool is raised up on a deck or above surrounding areas, then you can siphon it out. This is exactly the same way fish tank owners get water out of their tanks.
First get a garden hose that is long enough to end up in an area lower than the desired depth. Put the hose completely in the pool so it fills with water. Then seal both ends with either a clamp or a thumb, but make sure it is SEALED. Then take one end to the lower area and unclamp. Then wait for the water level to be lower than the plumbing.

(source: richmondaquarium.com)