An acoustic system for underwater object imaging, comprising an underwater transceiver antenna of electro-acoustic type, and control and processing means configured to carry out a detection step (block 61) and an imaging step (block 62); the detection step (block 61) including: • operating the underwater transceiver antenna to transmit isotropic acoustic pulses and to receive corresponding echo signals; • computing average powers of samples of said echo signals by using a sliding window of integration of squared amplitudes of the samples of said echo signals; • weighting the computed average powers by an exponential function of compensation for spherical divergence attenuation; and • detecting an underwater object (5;7) on the basis of the weighted average powers and of a detection threshold; the imaging step (block 62) including transmitting, by means of the underwater transceiver antenna and for each azimuth angle included in a predetermined range of azimuth angles, a corresponding acoustic pulse by: • using a fan beam pointed at said azimuth angle and having a predetermined width in azimuth and a predetermined width in elevation, wherein said predetermined width in elevation is greater than said predetermined width in azimuth; and • feeding the underwater transceiver antenna with excitation signals such as to cause a coherent recombination of corresponding acoustic signals transmitted in water at a three-dimensional focusing region located at a predetermined focusing distance from the underwater transceiver antenna, at which predetermined focusing distance an underwater object (5;7) is located; wherein the acoustic pulses are transmitted by varying azimuth pointing of the fan beam so as to perform a complete scan of the predetermined range of azimuth angles; said imaging step (block 62) further including: • for each acoustic pulse transmitted, receiving a corresponding echo signal by means of the underwater transceiver antenna; • processing the received echo signals by performing a beamforming method so as to compute a three-dimensional digital map of powers associated with a predetermined grid of points located in the three-dimensional focusing region; wherein the received echo signals are processed by forming pencil beams, each of which is pointed at a corresponding point of said predetermined grid; and • processing the three-dimensional digital map of powers by performing an edge detection method so as to form an image of the underwater object (5;7); the acoustic system being characterized in that the processing means are configured to: • compute a distance, from the underwater transceiver antenna, of an underwater object detected in the detection step; and • carry out the imaging step (block 62) by using said computed distance as the predetermined focusing distance. • operating the underwater transceiver antenna to transmit isotropic acoustic pulses and to receive corresponding echo signals; • computing average powers of samples of said echo signals by using a sliding window of integration of squared amplitudes of the samples of said echo signals; • weighting the computed average powers by an exponential function of compensation for spherical divergence attenuation; and • detecting an underwater object (5;7) on the basis of the weighted average powers and of a detection threshold; • using a fan beam pointed at said azimuth angle and having a predetermined width in azimuth and a predetermined width in elevation, wherein said predetermined width in elevation is greater than said predetermined width in azimuth; and • feeding the underwater transceiver antenna with excitation signals such as to cause a coherent recombination of corresponding acoustic signals transmitted in water at a three-dimensional focusing region located at a predetermined focusing distance from the underwater transceiver antenna, at which predetermined focusing distance an underwater object (5;7) is located; • for each acoustic pulse transmitted, receiving a corresponding echo signal by means of the underwater transceiver antenna; • processing the received echo signals by performing a beamforming method so as to compute a three-dimensional digital map of powers associated with a predetermined grid of points located in the three-dimensional focusing region; wherein the received echo signals are processed by forming pencil beams, each of which is pointed at a corresponding point of said predetermined grid; and • processing the three-dimensional digital map of powers by performing an edge detection method so as to form an image of the underwater object (5;7); • compute a distance, from the underwater transceiver antenna, of an underwater object detected in the detection step; and • carry out the imaging step (block 62) by using said computed distance as the predetermined focusing distance.