Aquaculture plays a crucial role in global food supply, but its environmental implications, especially in marine settings, remain a concern. While terrestrial aquaculture monitoring is robust, offshore facilities have not received similar scrutiny. Our study leverages the C-band wavelength of Sentinel-1 to detect and map offshore aquaculture facilities in Vietnam, a country where the ubiquitous presence of aquaculture activities pose an environmental risk. Using the current Sentinel-1 dataset, we were able to characterise unique backscattering patterns from fish cages and aquaculture boats. To distinguish aquaculture facilities from other structures in the sea, we conducted an object-based image analysis to highlight the distribution pattern of aquaculture facilities as opposed to other objects such as oil rigs or bridges. Next, we manually created a training dataset for a supervised classification, revealing a notable presence of these facilities in calm, bay-sheltered seas. Our findings underscore the need for enhanced marine aquaculture regulation and set the stage for exploring its environmental ramifications, particularly concerning water quality and pollution.