In　underwater　environments,　existing　capacitive　power　transfer　(CPT)　systems　only　consider　the　capacitance　or　conductivity　of　the　coupling　plate　when　analyzing　transfer　efficiency.　This　is　inconsistent　with　the　characteristics　of　the　underwater　coupler.　Thus,　it　would　lead　to　inaccurate　efficiency　models　and　difficult　efficiency　optimization　design.　To　address　the　above　issue,　this　paper　models　the　underwater　CPT　system　by　simultaneously　considering　the　capacitance　and　conductivity　of　the　coupler　in　the　efficiency　model,　so　a　more　accurate　efficiency　model　of　CPT　system　in　the　underwater　environment　is　derived.　On　this　basis,　the　efficiency　optimization　design　of　the　underwater　CPT　system　is　carried　out,　and　the　optimization　design　process　is　provided.　Finally,　the　accuracy　of　the　proposed　efficiency　model　and　the　effectiveness　of　the　design　method　have　been　demonstrated　through　simulation　and　experimentation.　With　the　proposed　optimization　method,　the　underwater　CPT　system　achieves　a　transfer　efficiency　of　82%　within　a　load　range　of　10　Omega　to　60　Omega　at　a　plate　spacing　of　100　mm.　The　maximum　transfer　efficiency　reaches　90%.