This　paper　considers　formation　control　in　dynamic　positioning　(DP)　of　multiple　offshore　vessels.　Toward　the　higher　accuracy,　reliability,　and　robustness　demands　in　the　advanced　marine　vessels,　we　shall　attempt　a　cooperative　robust　output　regulation　based　synthesis　for　the　problem.　It　is　shown　that,　when　the　vessels　modeling　parameters　in　question　are　unknown,　the　global　robust　asymptotic　formation　control　can　be　systematically　approached　in　a　distributed　control　fashion.　Specifically,　the　controller　can　be　constructed　by　incorporating　suitable　internal　models　and　performing　a　recursive　block　backstepping　design　for　the　augmented　system.　Hence,　it　assures　accurate　formation　control　with　robustness　against　model　uncertainties.