  We derive the special and general relativistic hydrodynamic equations of
motion for ideal fluids from a variational principle. Our approach allows to
find approximate solutions, whenever physically motivated trial functions can
be used. Illustrating this, a Rayleigh-Plesset type equation for the
relativistic motion of a spherical (QGP) droplet is obtained. Also the
corresponding general relativistic effective Lagrangian for spherically
symmetric systems is presented.
