We consider the robustness properties of a formation control system for marine surface vessels. Intervessel constraint functions are stabilized to achieve the desired formation configuration. We show that the formation dynamics is Input-to-State Stable (ISS) to both environmental perturbations that affect each vessel and to disturbances that affect the inter-vessel relationships e.g. communication noise. Next, we prove ISS of a formation where at least one vessel is in closed loop with, a class of position control laws, in addition to the formation control law. This class encompass control laws for point stabilization or path following. Hence, the designer can utilize previously developed controllers for single vessels in a formation control setting. A formation of three tugboats where one is in closed loop with a path following controller is simulated to verify the theoretical results.