Water is a mother liquid of life. To understand why water is indispensable to life, I have been investigating the structures and interactions at protein-water interface, i.e. the hydration structures of proteins, by cryogenic X-ray crystallography and molecular dynamics simulation. Through developing devices and experimental techniques in cryogenic X-ray diffraction experiments, I realized that cryogenic X-ray crystallography is one of techniques effectively observe hydration structures of proteins surface. In addition, the author developed a novel calculation codes to characterize systematically hydration structures of the protein structures determined at around 100 K, as to the amount of water molecules, the hydrogen-bond geometries, the local and the global distribution of them on the surface of proteins. The standard tetrahedral hydrogen bonds of water molecules in bulk water were retained in the hydration structures and contributed to extend three-dimensional network of hydrogen bonds among hydration water molecules and oxygen and nitrogen atoms of protein surface. In cryo-crystallography of multi-subunit or multi-domain proteins, the reorganization of hydration structure occurred with the dynamical motions of proteins. In that, water molecules act as inhibitors for protein motions, glue to stabilize the higher-order structures or lubricant to realize conformational changes by utilizing its tetrahedral arms of hydrogen bonds. Thus, hydration water molecules must have great influences on the dynamics and functions of proteins in aqueous solution.