In　a　multiple-sequences-spread-spectrum　(MSSS)　communication　system,　the　transmitted　signal　is　made　up　of　superimposed　cyclic　shift　sequences,　each　sequence　carrying　a　symbol,　so　the　data　rate　is　inceased.　In　this　context,　we　propose　an　in-phase/quadrature　index　modulation-aided　MSSS　system　(MSSS-I/Q-IM),　which　achieves　higher　data　rate　and　better　bit-error-rate　(BER)　performance.　The　new　system　maps　the　additional　bits　to　the　cyclic　shift　length　of　the　superimposed　sequences　of　I/Q　branches.　The　sequences　in　two　branches　carry　the　real　and　imaginary　parts　of　the　M-ary　phase-shift-keying　(MPSK)　symbols,　respectively.　We　derive　the　theoretical　BER　of　the　MSSS-I/Q-IM　system　over　an　additive　white　Gaussian　noise　(AWGN)　channel.　Then,　the　simulation　experiments　of　the　proposed　system　and　MSSS　system　over　an　underwater　acoustic　channel　are　analyzed　and　compared.　Finally,　field　experiments　are　carried　out　in　the　pool　environment.　The　experiment　results　show　that　the　proposed　scheme　can　achieve　higher　data　rate　or　better　BER　performance　via　adjusting　the　IM　parameter.　This　is　important　for　the　time-variant　underwater　acoustic　transmission　environments.