  This paper provides an upper-bound for the capacity of the underwater
acoustic (UWA) channel with dominant noise sources and generalized fading
environments. Previous works have shown that UWA channel noise statistics are
not necessary Gaussian, especially in a shallow water environment which is
dominated by impulsive noise sources. In this case, noise is best represented
by the Generalized Gaussian (GG) noise model with a shaping parameter $\beta$.
On the other hand, fading in the UWA channel is generally represented using an
$\alpha$-$\mu$ distribution, which is a generalization of a wide range of well
known fading distributions. We show that the Additive White Generalized
Gaussian Noise (AWGGN) channel capacity is upper bounded by the AWGN capacity
in addition to a constant gap of $\frac{1}{2} \log \left(\frac{\beta^{2} \pi
e^{1-\frac{2}{\beta}} \Gamma(\frac{3}{\beta})}{2(\Gamma(\frac{1}{\beta}))^{3}}
\right)$ bits. The same gap also exists when characterizing the ergodic
capacity of AWGGN channels with $\alpha$-$\mu$ fading compared to the faded
AWGN channel capacity. We justify our results by revisiting the sphere-packing
problem, which represents a geometric interpertation of the channel capacity.
Moreover, UWA channel secrecy rates are characterized and the dependency of UWA
channel secrecy on the shaping parameters of the legitimate and eavesdropper
channels is highlighted.
