This paper presents an embedded geofencing system for monitoring vessel approach to the protected area surrounding Ilha dos Lobos (Torres, RS). The platform integrates an ESP32-based data logger, a GPS receiver and an on-board Haversine distance calculation to detect vessel proximity relative to a configurable protection radius. Position fixes and computed distances are persistently recorded to on-board flash (LittleFS) and events of entrance/exit are timestamped for later analysis. In laboratory and on-land trials, the system proved robust: GPS decoding, real-time distance computation and local logging operated reliably under continuous readout, and the onboard alert (buzzer) correctly signaled state changes. Limitations observed in controlled tests, including the need for field trials to evaluate GPS degradation and communication loss. A responsive web platform was developed to support remote visualization, alerting, and historical data analysis. Finally, are presented implementation details to facilitate replication and deployment on small craft for control and environmental monitoring.