Abstract: An underwater detection system for displaying underwater conditions over a large area around a ship comprises a linear array type transducer, a transducer training mechanism for rotating the transducer stepwise in a horizontal plane, a receiver which forms a receiving beam for scanning a fan-like vertical cross-sectional area by giving specified time delays or phase differences to echo signals received by the transducer, an image memory for storing echo signal data picked up by the receiving beam in addresses defined by horizontal directions relative to the ship and depths, a sampling circuit for reading out data of one vertical line after another in a downward direction from the image memory and determining a bottom depth in each vertical line, a seabed data memory for storing bottom depths, a PPI image generator for producing a fractional PPI image depicting the bottom based on bottom depth data read from the seabed data memory, and an indicator for presenting search results in the form of a plan view cover