The possible cruising distance of the electric boats is still short compared with that of similar-sized internal combustion engine boats. Currently, electric boat operators have to estimate the possible cruising distance from weather (wind direction or power), wave (height and tide), and battery (voltage, current, state of charge) conditions. Battery failure has to be avoided from the viewpoint of operational safety. In order to solve this problem, a Navigation Support System (NSS) that provides the possible cruising distance and the recommended speed, and restricts the output power to achieve the desired operation, was developed. In this paper, in order to realize this system, an estimation algorithm and an output power restriction algorithm based on the analysis of characteristics of the boat, the motor and the battery discharge is derived. Verification tests in actual seas confirm the system’s usability.