A new system for submarine application of cryo-cooled SQUID (superconducting quantum interference device) is introduced. Those sensors are mounted on a towed underwater platform and allow the passive measurement of the full magnetic gradient tensor information. The depressor based platform carries a pressurized hull, containing all important components for the underwater operation of a liquid Helium cooled system at ambient pressure. The most important component is a special designed Dewar which handles the reservoir for the cryogen liquid and simultaneously provides the operating conditions of the SQUID sensors by thermal and electromagnetic shielding. The electronics for power supply, control and readout of the sensors is also placed inside the pressurized hull. Since a dipole localization algorithm requires knowledge about localization and orientation of the system during the entire measurement time, a combination of a GPS on the ship together with ultra-short baseline (USBL) is realized. The evaporated Helium gas is released to the surrounding water using a compressor. All components are designed for low-distortion operation to support the applied sensors which have a sensitivity of below 100 fT/(m*sqrt(Hz)) under field conditions and operate at a sampling rate of 1 kHz. These properties are supported by an innovative data processing that eliminates measurement artefacts out of the acquired signals and offers a trustable fundament for our advanced inversion algorithms. These algorithms use the properties of the sensors as well as a well-founded theory to allow the localization of various objects with high accuracy.