​​To investigate unexplored areas and apply methodologies suitable for monitoring volcanic activity, a pioneer study was undertaken in the crater lagoon of Vila Franca do Campo Islet to identify possible CO₂ degassing zones in the marine environment. This study was carried out using a foating accumulation chamber, which to the best of our knowledge, has only been applied to lacustrine environments. A survey was carried out in the crater lagoon in August 2017, resulting in a total of 143 CO₂ fux measurements. Other parameters, such as water temperature, pH, electrical conductivity, and depth, were also determined at each measuring point, covering a study area of 0.02 km². At one of the deepest sites with higher CO₂ fux values, water samples were collected at depth for determination of stable isotopic composition. Water temperatures ranged between 21.0 and 27.0 °C, pH from 8.24 to 8.89, and electrical conductivity from 52.5 to 53.9 mS/cm. As expected, due to the marine water composition, sampled waters are of the Na-Cl type. CO₂ fux varied between 0.581 and 1.0 g m^-2 d^-1 (average = 2.967 g m^-2 d^-1). A single CO₂ population, characterized by low CO₂ fuxes, points to a biogenic CO₂ origin (δ¹³C = −9.62‰). The estimated value for the total CO₂ emitted from the water surface is 0.05 t d^-1. CO₂ fux measurements were also compared with the structural features that cross the tuf cone, but no clear relation was observed with fractures/faults. This methodology should be applied to other coastal marine areas, especially where anomalous gas emissions have been reported.​