This paper investigates the waterproofing properties of two water swelling gasket materials, i.e. water-swelling rubber (WSR) and water-swelling polyurethane (WSP). Swelling tests were performed to measure the swelling ratios in both compression and free swelling states. A dynamic mechanical analyzer (DMA) was employed to measure the storage moduli under different compression pressures and in different saline environments. Scanning electron microscope (SEM) was performed to investigate the micro-damage morphology for both WSR and WSP. According to the test results, under the compression load of 1.0 MPa, the maximum axial swelling ratio (S) and free swelling ratio (S) of WSP was nearly four times and twice that of WSR, and the storage modulus E’ of WSP was three times that of WSR. Also, the micro-cracks of WSP were much less and shallow than those of WSR under the same load condition and saline environments. Thus, the WSP specimen showed better water-proof performance than the WSR specimen under the complex submarine condition.