  Optical wireless communication is a promising technology for underwater
broadband access networks, which are particularly important for high-resolution
environmental monitoring applications. This paper focuses on a deep sea
monitoring system, where an underwater optical wireless network is deployed on
the seafloor. We model such an optical wireless network as a general queueing
network and formulate an optimal relay placement problem, whose objective is to
maximize the stability region of the whole system, i.e., the supremum of the
traffic volume that the network is capable of accommodating. The formulated
optimization problem is further shown to be non-convex, so that its global
optimization is non-trivial. In this paper, we develop a global optimization
method for this problem and we provide an efficient algorithm to compute an
optimal solution. Through numerical evaluations, we show that a significant
performance gain can be obtained by using the derived optimal solution.
