Our research group aims to realize a low-cost alert system that informs the crews of small-sized ships of approaching floating objects on the sea. We previously proposed a maritime object identification system that uses a Radio Frequency Identification (RFID) tag. However, we concluded that it is difficult to measure the received signal strength, due to the effects of waves and other factors, on the sea, and the difficulty in miniaturizing the transmitter and the receiver devices. In this paper, we discuss the construction of a high-precision positioning system necessary to measure a received signal strength on the sea. We decided to use a ESP32, a low-power-consumption Wi-Fi controller, to elucidate characteristics of the received signal strength of Wi-Fi on the sea, which is necessary for a proximity warning system. Finally We make a prototype of the proximity warning system.