Everything you always wanted to know and never dared to ask about hydrodynamics. But only what is really needed to subsequently understand the theory and practice of waves. For instance, differences between Lagrangian and Eulerian approaches to the fluid motion, and their use for the erosion of a particular place or the following of an oil spill respectively, are presented. Moreover, laws of conservation of mass and momentum are explained in a simple way along with the consequent Navier–Stokes equations and the precise simplifications to obtain simpler equations such as Euler's. Numerous examples and practical cases allow a clear understanding not only of the concepts but also of the fields of validity. Eventually, potential theory is applied to arrive at the Laplace and Bernoulli equations.