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In this study, we designed and observed an ultrabroadband absorber and bi-functional sensor based on a water-based metamaterial (MM). The cavity material of the unit cell was photosensitive resin, the internal water layer was formed by connecting two opposite cross-types and rectangles, and the bottom layer was a copper backplate. Simulation results exhibited that the absorption rate for the water-based MM was above 90% in the frequency ranges of 3.01–3.12 and 13.31–57.75 GHz. Experimental results were in good agreement with the simulation results. We observed and analyzed the distribution of electric and magnetic fields at the absorption peaks and determined that the physical mechanism was the strong magnetic resonance between the water layer and the surrounding photosensitive resin. Owing to the irregular shape of the water layer, the absorption properties of the proposed MM were sensitive to incidence and polarization angles. At a higher polarization angle of 45°, the corresponding absorption band changed to 3.28–4.34 and 13.31–57.75 GHz, covering the operating bands for 5G communication. The proposed water-based MMs were also sensitive to the refractive index and have two detection frequencies. The amplitude and frequency variations with different cover materials can be sensed. The sensitivities corresponding to the two resonance points were 25.58 dB/RIU and 5.04 GHz/RIU and 27.85 dB/RIU and 4.09 GHz/RIU, respectively, exhibiting good sensing performance. Thus, the designed water-based MMs have potential applications in 5G signal shielding and biological detection.