Biomass-derived activated carbons (ACs) are promising sustainable adsorbents for water polishing; however, their performance in real aqueous matrices cannot always be predicted from BET surface area alone. In this study, chemically activated biomass-derived carbonaceous adsorbents were prepared from Cocoa Pod Husk (CPH), Watermelon Peel (WP), and Pineapple Crown (PC) and evaluated for Chemical Oxygen Demand (COD) removal from real eutrophic lagoon water. The materials were characterized by N₂ adsorption–desorption analysis, including BET surface area and BJH pore-size assessment, XRD, Raman spectroscopy, FTIR, UV–Vis diffuse reflectance spectroscopy, and pH_PZC analysis. Although all adsorbents exhibited low N₂-BET surface areas, adsorption performance was governed by apparent functional pore accessibility inferred from adsorption behavior, pore size distribution, surface chemistry, structural disorder, electronic delocalization, and surface charge. Among the acid-activated samples, ACPCSA5 showed a narrow average pore size of 1.720 nm and achieved near-complete COD removal. Its superior performance was associated with oxygen-containing functional groups, partially developed sp² carbon domains, lower optical band gap, BJH-derived pore architecture, and favorable surface charge at lagoon pH. The Microbial Regrowth Potential Index (MRPI) was introduced only as a conservative COD-based proxy, not as a validated biological indicator. Overall, this work demonstrates that adsorption in real-water matrices depends on accessible pore architecture and multifunctional surface chemistry rather than BET surface area alone.