  We show that many hydrodynamical properties of the BEC/BCS crossover in the
presence of a Feshbach resonance at T=0 can be derived easily from the
derivative expansion of the (exact) fully renormalized one-loop effective
action. In particular, we calculate the velocity of sound throughout the BCS
and BEC regimes and derive the generalized superfluid continuity equations for
the composite two-fluid system.
