We describe a technique for high definition marine coastal survey in which we have used to locate possible artificial magnetic sources, such as wrecks, in the Eastern Ligurian Sea. The data were recorded with a marine proton magnetometer using a high density grid. The accuracy in grid location was measured by a network of three coastal position stations, transmitting in 5.48 GHz band. Magnetograms obtained from a geomagnetic observatory, located in a magnetically quiet area on the Isle of Tino, less than, 0.5 km from the east edge of the survey area, were used for the time reduction of the marine survey data. By this method the crossover errors were reduced to the instrumental accuracy level (1 nT). The local magnetic anomaly due to a source on the sea bottom (at about 35-70 m depth in the explored area) can be deduced from the high frequency component of the spectral analysis of the corrected and reduced to the pole data. A map of the field derivative was calculated. The result evidences a signal from a possible small artificial source, equivalent to several tons of iron, at a location where a subsequent submarine exploration demonstrated the presence of a small wreck.