This idea is widely believed, but I think it is not entirely complete in its description of the process. Degrading organics can add CO2, and added CO2 lowers pH. If that is what you mean, that's perfect. CO2 and carbonate alkalinity together mathematically determine the pH. But if the carbonate alkalinity is normal, the only other factor in determining pH is the CO2 level in the water. "Acidic substances" do not lower pH in seawater just by being there. They may reduce alkalinity when first added, and some may even mislead aquarists in determining the relationship between total alkalinity and carbonate alkalinity, but they are not lowering pH (unless they degrade to CO2).