A large fraction of cell and tissue mass is made of water. The flow of water across the cell surface follows osmotic and hydraulic pressure gradients, and is actively controlled by the cell. This physical fact suggests that the mechanical behavior of cells is intimately connected with cell ionic homeostasis and osmotic control. In this talk, we will describe some of our recent experimental and modeling work on water dynamics in cells. We explore how cells control their cytoplasm water content and therefore the cell size. We will describe how cells can use directional flow of water to propel themselves and migrate, and in the case of epithelia, pump fluid between tissue compartments. In the later case, epithelial pumping of water can strongly influence tissue shape and morphogenesis.