Several works are aiming to develop techniques allowing detecting False Data Injection Attacks, which represents one of the most harmful attacks due to its ability to damage a Cyber Physical Systems (CPS). Among these techniques the watermarking represents one of the most used ones. This paper proposes the design of a False Data Injection Attack (FDIA) against a CPS protected by a watermark-based detector. The attack herein proposed is achieved in two phases. The first one is a passive phase, where the adversary builds a black box model of the system. Then, he uses the already built model to create the FDIA without being detected by the watermark based detector. The extensive simulations prove that this attack could be used to deceive the system even with the presence of a dynamic watermark.