Hydrodynamics provides a universal low-energy effective description of interacting many-body systems. Traditionally, it is formulated in terms of equations of motion derived from the relevant conservation laws. However, this classical framework neglects fluctuations of hydrodynamic observables required by the fluctuation–dissipation theorem (FDT). The Schwinger–Keldysh effective field theory (SK EFT) offers a Wilsonian, action-based formulation of hydrodynamics that systematically incorporates such fluctuations. In this approach, the effective action is generically non-unitary (complex), encoding macroscopic dissipation, while the FDT is implemented through a discrete Kubo–Martin–Schwinger (KMS) symmetry. This symmetry also underlies the emergence of the second law of thermodynamics within hydrodynamics.