  The goal of this paper is to describe Zermelo's navigation problem on
Riemannian manifolds as a time-optimal control problem and give an efficient
method in order to evaluate its control curvature. We will show that up to
change the Riemannian metric on the manifold the control curvature of Zermelo's
problem has a simple to handle expression which naturally leads to a
generalization of the classical Gauss-Bonnet formula in an inequality. This
Gauss-Bonnet inequality enables to generalize for Zermelo's problems the E.
Hopf theorem on flatness of Riemannian tori without conjugate points.
