Depth Sounders, echo sounders, or fathometers should be the second piece of equipment acquired before conducting a search in the ocean.  It is necessary, not only to indicate the depth, but also to prevent the survey or salvage vessel from becoming a shipwreck statistic itself in shallow, reef or rock-strewn waters.  It helps locate shoals, rocks, and reefs where ships may have wrecked.  Many of the modern depth sounders are used by fishermen to locate reefs and rocks where fish may be more plentiful.  Some of them produce an image of the bottom showing a great deal of detail.  In the past decade, two of the richest Chinese porcelain wrecks, both lying between 120 and 140 feet deep, were located using only a depth sounder.