Summary: Nonlinear feedback control laws for controlling multiple robotic vessels in arbitrary formations are proposed. The presented leader-follower formation control approach uses only the inertial information obtained from the immediate neighbours of each vehicle via communication for control calculations. A three-degree-of-freedom (3DOF) surface vessel dynamic model and the method of Lyapunov has been used to derive the nonlinear control laws that stabilize the relative distance and orientation of neighboring vessels. It is shown that the internal dynamics of the 3DOF vessel as an underactuated system is also stable. The performance of these control laws is demonstrated in the presence of sea disturbances by computer simulations using a 6DOF dynamic model of the surface vessel. These controllers can be utilized to control an arbitrary number of robotic vessels moving in very general formations.