  We introduce equations of motion for spin dynamics in a ferromagnetic
Bose-Einstein condensate with magnetic dipole-dipole interaction, written using
a vector expressing the superfluid velocity and a complex scalar describing the
magnetization. This simple hydrodynamical description extracts the dynamics of
spin wave and affords a straightforward approach by which to investigate the
spin dynamics of the condensate. To demonstrate the advantages of the
description, we illustrate dynamical instability and magnetic fluctuation
preference, which are expressed in analytical forms.
