## 2 Optical Transmission Characteristics of Seawater  
Quantum key distribution can be realized by utilizing the \"optical transmission window\" of seawater to the blue and green light. However, it is difficult to make a detailed analysis of its transmission characteristics because of the complex composition of seawater and the influence of many factors such as environment and region. Here, we mainly take the influence of optical absorption and scattering on the performance of the CV-QKD system into account. Absorption leads to the loss of light intensity, and scattering causes the deflection of light from its original direction [22], which is similar to beam-wander in the atmosphere [10]. Due to the existence of these two factors, the energy of the signal light that can be received by the receiver is continuously decaying. The absorption coefficient \(a\) and the scattering coefficient \(b\) are both functions of the optical wavelength \(\lambda\) and the chlorophyll concentration \(C\), and they satisfy the following equation [20; 21; 22; 23]\[a(\lambda) =[a_{w}(\lambda)+0.06a_{c}(\lambda)C^{0.65}][1+0.2e^{-0.014(\lambda-4 40)}],\] \[b(\lambda) =0.3\frac{550}{\lambda}C^{0.62}, \tag{1}\] \[c(\lambda) =a(\lambda)+b(\lambda),\]  
where \(a_{w}(\lambda)\) is the absorption coefficient of pure water, \(a_{c}(\lambda)\) is a nondimensional, statistically derived chlorophyll-specific absorption coefficient, and \(c(\lambda)\) is the total attenuation coefficient. Without loss of generality, we mainly analyze four typical types of seawater, namely pure sea water, clear ocean water, coastal ocean water, and turbid harbor water, and the complexity of these four types of seawater increases gradually. The relationship between the attenuation coefficients of the four different types of seawater and the wavelength of the signal light is shown in Figure 1. As expected, the absorption and scattering coefficients of the four types of seawater to the signal light gradually increase with the complexity of seawater. It can also be seen in the figure that the scattering coefficient \(b\) decreases obviously as the wavelength increases, and this trend becomes more obvious with the increase of the complexity of the seawater. In addition, the scattering coefficient of turbid harbor water is much higher than the three other types water. By contrast, the light absorption coefficients of the four types of water are relatively stable, fluctuating between 0 and 0.7. Utilizing the optical window effect of seawater on blue-green light with a wavelength of 450-550 nm, we select the light with a wavelength of 520 nm to study the influence of seawater on the secure transmission distance of the secret key in a CV-QKD system. The corresponding attenuation coefficients can be obtained through the simulation shown in Figure 1, and the values of each attenuation coefficients are listed in Table 1[21; 22; 24]. Without loss of generality, light with a wavelength of 1310 or 1550 nm is usually used as the signal light in standard single-mode fiber communication, and the corresponding attenuation coefficients are 0.34 and 0.21 dB/km, respectively [25; 26]. In comparison, the attenuation coefficients of the optical signal propagating in the water channel are much larger than that in the fiber channel. As a result, the secure transmission distance of the secret key in a CV-QKD system mediated by a seawater channel is reduced by several orders of magnitude. In the case of such short distance communication, we can assume that the seawater channel is a linear attenuation model [20]. Similar to the optical fiber communication, transmittance of the seawater channel can be calculated by \(T_{sea}=e^{-cL}\), where \(c\) represents the total attenuation coefficient and \(L\) is the transmission distance.