Water alone will not remove grease, and the reason is a matter of shape.

A soap molecule has two ends that behave differently. One end is attracted to water; the other is attracted to oil and repelled by water. Drop soap into greasy water and the molecules arrange themselves automatically — oil-loving ends buried in the grease, water-loving ends facing out.

The grease ends up wrapped in tiny balls with a water-friendly surface, and the water can now carry it away. Nothing was dissolved and nothing was killed. The grease was simply given a coat that water is willing to hold on to.