The　rise　in　demand　for　underwater　wireless　communication　networks　(UWCN)　has　been　driven　by　the　emergence　of　new　applications　including　unmanned　underwater　vehicles,　deep-sea　exploration,　maritime　and　underwater　archaeology　research,　and　diver　communications.　For　various　applications,　underwater　network　lifetime　must　be　long,　since　unlike　terrestrial　sensor　networks,　it　is　a　major　job　to　change/recharge　node　batteries　in　underwater　environments.　In　this　paper,　we　introduce　a　solution　where　critical　nodes　in　a　UWCN　are　periodically　recharged　by　small　renewable　energy　sources.　Further,　a　caching　mechanism　is　introduced　to　relieve　critical　nodes　of　heavy　workload　when　their　residual　energies　run　low.　By　leveraging　these　two　techniques　of　energy　replenishment　and　content-caching,　the　network　lifetime　extension　challenge　is　formulated　as　an　optimization　problem.　The　results　indicate　that　our　proposed　solution　offers　up　to　45%　longer　UWCN　lifetime　compared　to　existing　solutions.