Considering the broadening of the laser pulse time delay caused by multiple scattering in ocean channels, we investigate the modeling of underwater wireless laser pulse responses using Monte Carlo simulations and Gamma functions. Seawater optical characteristics are analyzed, and a closed-form expression of a multi-Gamma function is proposed to simulate the impulse response in underwater channels. The theoretical analysis and modeling results show that the model uses four Gamma functions to represent laser pulse transmission under water, i.e., four paths with different lengths generated owing to the difference in the scattering intensity. The first three paths are mainly quasi-ballistic light paths with low scattering series and short optical path, whereas the last one presents a high scattering series and multiple scattered light. Simultaneously, the model accuracy and superiority are verified via comparison, and the proposed method is used to simulate the impulse response of laser transmission under different system parameters. The obtained results well describe the laser pulse scattering characteristics and time delay broadening under water.