The rapid growth of cities along coastlines has increased the need for maritime structures, highlighting the importance of studying biofouling on floating infrastructure. Biofouling, the buildup of organisms on underwater surfaces, can significantly impact the stability, lifespan, and expenses of these structures. We conducted a thorough numerical analysis to replicate the water quality of Tokyo Bay in 2000 and study the biofouling dynamics on a large floating platform at Yokosuka Shipyard in Yokosuka City. Our study utilized a specialized biofouling sub-model to simulate the conditions around the mega-float during that time. We used numerical simulations to reconstruct water quality parameters, which closely aligned with observed data, ensuring the reliability of our findings. We studied biofouling dynamics under four scenarios with varying mortality rates, a key variable with significant uncertainty. Our results indicated minimal differences in environmental effect of biofouling dynamics across these scenarios, suggesting that the current model may be too simplistic and not adequately account for complex interactions such as organism detachment. This finding underscores the need for further development of our biofouling models to better understand and mitigate the impacts on maritime structures. The insights from our study provide valuable information for improving the design, operation, and maintenance of future floating infrastructure in rapidly developing coastal areas. This research not only contributes to our understanding of biofouling impacts but also aids in the sustainable development of marine infrastructure.