The　challenges　associated　with　acquiring　the　clear　images　of　objects　in　underwater　environment　are　difficult　to　overcome　due　to　the　absorptive　and　scattering　nature　of　seawater.　Recently,　the　research　community　has　focused　on　mitigating　these　effects.　The　recent　developments　in　image　enhancement　algorithms　and　strategies　of　signal　light　enhancement　have　brought　improvement　in　some　application　areas.　In　this　work,　we　review　the　six　most　common　methods　based　on　signal　light　enhancement.　We　present　the　individual　working　mechanisms,　latest　representative　advances,　and　suitable　application　conditions.　Moreover,　we　also　present　a　detailed　comparison　of　these　techniques.　In　each　technique,　we　present　their　applicable　environments　and　conditions　according　to　the　following　indicators:　operating　distance　(from　2　attenuation　lengths　(AL)　to　13.5　AL),　resolution　(from　centimeter　to　millimeter),　and　field　of　view　(FOV).　By　summarizing　and　analyzing　the　existing　problems　that　restrict　the　underwater　optical　imaging　techniques,　the　future　development　trends　are　prospected.