Various　image　enhancement　algorithms　are　adopted　to　improve　underwater　images　that　often　suffer　from　visual　distortions.　It　is　critical　to　assess　the　output　quality　of　underwater　images　undergoing　enhancement　algorithms,　and　use　the　results　to　optimise　underwater　imaging　systems.　In　our　previous　study,　we　created　a　benchmark　for　quality　assessment　of　underwater　image　enhancement　via　subjective　experiments.　Building　on　the　benchmark,　this　paper　proposes　a　new　objective　metric　that　can　automatically　assess　the　output　quality　of　image　enhancement,　namely　UWEQM.　By　characterising　specific　underwater　physics　and　relevant　properties　of　the　human　visual　system,　image　quality　attributes　are　computed　and　combined　to　yield　an　overall　metric.　Experimental　results　show　that　the　proposed　UWEQM　metric　yields　good　performance　in　predicting　image　quality　as　perceived　by　human　subjects.