Water can contain dilute chemical solutions that can dissolve rock, such as hydrochloric acid (HCl) working against limestone (CaCO3). Chemical weathering.
(2) Water can carry physical particles of small grains, and when it runs across another rock, this acts like a natural sandpaper. Most river systems are carved this way.
(3) Liquid water can slip into cracks and fissures of rocks, really big rocks, and then when the temperature falls below freezing, the liquid water turns to ice. When this happens, the water expands by about 8% of its original volume, causing a "pushing" force outward on the rock. Over time, this freeze-thaw-freeze-thaw pumping will slowly break off chunks of rock as mass wasting (rockfalls).