Hydrodynamics is the dynamics of liquids, i.e. of low compressible fluids. It is part of rheology, a general science of fluids. For a while this entry will have a redirect fluid dynamics.
L. D. Landau, E. M. Lifschitz, Fluid mechanics (Russian original is Hydrodynamics), vol. 6 of the Course of Theoretical Physics, Pergamon Press
Vladimir I. Arnold, Boris A. Khesin, Topological methods in hydrodynamics Applied Mathematical Sciences 125, Springer
Dennis Sullivan, Algebra, topology and algebraic topology of 3D ideal fluids, arxiv/1010.2721 (has some ideas on need of a version of higher geometry at the end of the article)
wikipedia: fluid dynamics