An increasing number of underwater construction projects demand a solution for precise and highly resolved infrastructure monitoring which cannot be addressed by conventional sonar systems. We present an optical ranging system based on the pulsed time-of-flight method incorporating a signal-processing toolchain especially equipped for mapping technical infrastructure. This allows us to handle multiple returns from objects which are only separated by a 15 cm gap. A high lateral resolution of 4 mm in combination with a high measurement rate of 50 000 measurements per second allows the reconstruction of fine details. The measurement range can reach several 10 m, depending on the water quality. Further, we present the system design of our scanner and discuss first results obtained in an underwater measurement range.