When the energy is transfered into the water it creates waves. With more enegy, it will make waves with a larger amplitude and a larger frequency. With less energy, it will make waves with a smaller amlitude and a smaller frequency. When diving into a pool, if the diver is more aerodynamic then they can reduce the amout of splash and waves produced when the hit the waters surface.
Gravity is also pulling down on the swimmers causeing them to sink at rest. Swimmers also must keep there lungs full to stay afloat and they must use energy in their muscles to fight gravity, although in water it is not as effective because the human bosy is 75% water.