An　advanced　light　source　at　blue-green　waveband　is　the　key　to　improve　the　link-distance　and　communication　performance　of　an　underwater　wireless　optical　communication　(UWOC)　system.　This　work　presents　the　underwater　channel　characteristics　and　UWOC　system　using　a　blue　semiconductor　disk　laser　that　passively　mode-locked　by　a　semiconductor　saturable　absorb　mirror　for　the　first　time.　The　attenuation　coefficients　of　the　continuous-wave　and　mode-locked　lasers　under　different　concentrations　of　Maalox　solution　are　studied,　and　the　results　indicate　that　the　attenuation　coefficients　of　the　mode-locked　laser　are　obviously　lower　than　that　of　the　continuous-wave　laser.　Then　an　UWOC　system　based　on　an　acousto-optic　modulator　are　constructed,　and　the　water　tank　communication　experiments　are　carried　out　by　using　quasi-cyclic　low-density-parity-check　coding　and　64-pulse-position-modulation.　Experimental　results　show　that　the　bit　error　rates　of　the　mode-locked　laser　are　significantly　lower　than　that　of　continuous-wave　laser　under　the　same　Maalox　solution　concentration　and　same　signal-noise-ratio.　These　results　provide　an　attractive　light　source　for　practical　long-distance　UWOC.