Owing to the lack of a particular function to fit the channel impulse response (CIR) curves in clear ocean water, a method of fitting CIR curves of clear ocean water using double exponential function is proposed in this article. The proposed double exponential function is compared with four other well-known fitting functions—single Gamma, inverse Gaussian, double Gamma, and CEAPF—to show its superiority. Moreover, we compare and analyze the fitting performance and working range of these five functions under different conditions of seawater quality, link range, and field of view (FOV) of the receiver, and these are verified by Monte Carlo simulation experiment. Both theoretical analyses and simulation results suggested that the proposed double exponential function could achieve an accurate fitting performance not only in clear ocean water but also in harbor water and could outperform the four other functions, especially in a short link range of clear ocean water (e.g., 10 m) and in harbor water case with different link ranges and FOVs. From the results, the single Gamma function used to match the CIR curves in the atmospheric environment could also be used to fit the ones in clear ocean water and harbor water, whereas the inverse Gaussian function deviated the CIRs under any working case with larger errors; the double Gamma function could fit the CIR of coastal water and harbor water but could not fit that of clear ocean water; the CEAPF function could generally fit well with the CIRs in the three ocean water qualities and obtain good fitting results.