Sperm cells do move, but they aren't strong enough to create a pressure differential large enough to move semen. Fish in a pond don't move the pond.
The sperm travel(swim) through the semen(cum). The semen protects the sperm from the natural defenses of a womans body. If you have a low semen to sperm proportion you will be infertile as the sperm will be killed through a chemical reaction in the vag