Blue energy or salinity difference energy takes advantage of the free energy released in a mixture of two solutions with different salinity concentrations as it happens continuously in river mouths. Among the large number of available techniques that aim to harness blue energy, capmix (or capacitive mixing) methods allow to directly extract electrical energy without the need of any electromechanical converter such as turbines or heat engines. The main goal of this article is to analyze the potential of blue energy by capmix methods in Central America. So far, blue energy studies have been principally carried out in countries from the global North. Therefore, we describe experimental results with real sea and river waters from the Gulf of Fonseca, an area of special interest due to its hydrographic richness, which is situated among Honduras, El Salvador, and Nicaragua. An electrochemical cell, which consists of a pair of activated carbon electrodes coated with cationic and anionic polyelectrolyte layers, respectively, is used in the experiments. The cell voltage in open circuit (OCV) is used as a measure of the performance of the capmix process. It is found that the OCV is larger when natural river water is used instead of low salinity NaCl solutions. The rainy season in which the experiments were performed reduced the ionic content of the river, increasing the salinity difference with ocean waters. The feasibility of capmix as a means of clean energy production is discussed.