  This paper presents an analytical model and a geometric numerical integrator
for a system of rigid bodies connected by ball joints, immersed in an
irrotational and incompressible fluid. The rigid bodies can translate and
rotate in three-dimensional space, and each joint has three rotational degrees
of freedom. This model characterizes the qualitative behavior of
three-dimensional fish locomotion. A geometric numerical integrator, refereed
to as a Lie group variational integrator, preserves Hamiltonian structures of
the presented model and its Lie group configuration manifold. These properties
are illustrated by a numerical simulation for a system of three connected rigid
bodies.
