This study presents a detection method for mechanically fabricated defects on underwater steel pipes, using ultrasonic guided waves. Three different diameters (60, 90, and 114 mm) of 1000-mm long steel pipes were considered, along with several experimental design factors such as incident angles, incident distances, and the degrees of defects, to investigate how these factors affected the experimental results - the detectability of the mechanical defects. From the experimental results, we determined that the amplitude and arrival time of the first received wave signals gave a promising clue for distinguishing the existence of the defects and their severities. Between the amplitude and arrival time, the arrival time gave a more promising indication since it was affected by the experimental factors in a constant manner. Therefore, it was shown that the use of ultrasonic guided waves for underwater pipe inspection is feasible.