This paper studies the precision, and the accuracy, of an underwater target tracking system, using range-only and single-beacon methods, in shallow waters environments. For this study, different field tests have been realized in the OBSEA test site, a well-known and monitored area at 4 km from the coast and at 20 meters of depth, in the Mediterranean Sea (Barcelona). The tests have been conducted using two acoustic underwater modems from the company LinkQuest Inc. Moreover, the autonomous underwater vehicle developed by the Universitat Politècnica de Catalunya (called Guanay II) have also used to perform the tests.