Rising sea levels and intensified storm waves due to climate change may increase the wave loading and overtopping on vertical seawalls, potentially causing damage or coastal flooding. Porous submerged breakwaters are being used to retrofit these existing seawalls, protecting them from increased wave action. A detailed numerical modeling study was conducted to assess the performance of porous submerged breakwaters in reducing wave forces and overtopping when positioned at varying pool lengths. The volume-averaged Reynolds-averaged Navier–Stokes equations describe the behavior of porous media. This study focuses on the impact of pool length on the hydrodynamic performance of vertical seawalls under random wave conditions. Sixty-four cases were analyzed, comprising four significant wave heights, four peak periods, and four pool lengths. Each test case involved ∼1000 pseudo-random waves generated using the Joint North Sea Wave Project spectrum. The study reveals that porous submerged breakwaters effectively reduce wave forces and minimize overtopping on vertical seawalls. The findings show a non-monotonic variation in the reduction of wave forces and overtopping as the pool length ratio (lp/d) changes from 1.7 to 4.2. Notably, including porous submerged breakwaters results in a 100% reduction in overtopping over the seawall for lower relative wave heights (Hm0/d ≤ 0.17). Overall, the study observed a decrease in overtopping, ranging from 82% to 100% and 47% to 69% in forces, for the given wave parameters and pool lengths. Furthermore, this study presents empirical formulations for estimating wave-induced forces and overtopping on seawalls retrofitted with porous submerged breakwaters.