White　light-emitting　low-dimensional　organic-inorganic　hybrid　lead　halide　perovskite　materials　have　been　considered　as　candidates　for　next　generation　of　light　sources　because　of　the　potential　in　daily　lighting　acting　as　white　light　emitters　or　phosphors.　However,　due　to　the　instability　of　perovskite　materials　in　water,　the　underwater　application　scenario　has　not　been　well　developed.　In　this　work,　we　report　a　method　employing　an　ultraviolet　(UV)-polymerizable　acrylic　monomer　to　wrap　a　low　dimensional　white　light　emitting　perovskite　single　crystal　(DMEDAPbBr　$_{\text{4}}$　)　powders　in　the　polymer　matrix.　We　investigate　the　exciton　dynamics　in　this　composite　material　by　transient　absorption　spectra.　More　importantly,　white　light-emitting　devices　(WLEDs)　are　manufactured　employing　this　composite　material　as　phosphor　excited　by　UV-LED.　Thanks　to　the　protection　of　the　polymer　matrix,　the　stability　against　the　water　of　this　WLED　is　very　good　allowing　the　devices　to　work　underwater　for　many　days.　This　strategy　in　this　work　illustrates　a　facile　way　for　the　application　of　white　light　emitting　low　dimensional　perovskite　single　crystals　in　WLEDs.　IEEE