Thermoplastic polyurethane (TPU) elastomers are frequently used in underwater applications. Desirable polymeric materials have lower densities and lower swelling ratios for seawater and paraffin oil, along with better mechanical strength than TPU after impregnation. To fabricate desirable composites for underwater applications, hollow glass microspheres (HGMs), TPU prepared from poly(tetramethylene glycol) (PTMG), and methyl diphenyl diisocyanate (MDI) were used. TPU-grafted hollow glass microspheres (TPU-g-HGM) were prepared to fabricate a composite that has better interfacial adhesion between the TPU matrix and the HGMs. The tensile strength of the composite increased with increasing HGM content, whereas the swelling ratio and density of the composite decreased with increasing HGM content. At fixed HGM content in the composite, the TPU/TPU-g-HGM composite exhibited a lower swelling ratio and better tensile strength than the TPU/HGM composite. In addition, the TPU/TPU-g-HGM composite exhibited enhanced mechanical strength as compared to TPU after being impregnated with seawater and paraffin oil.