This article presents the design, simulation and comparison between two controllers, each based on the PID and sliding mode control (SMC) strategies, of an AUV that is tracking a square trajectory while maritime disturbances and mass addition are present. For this purpose, a cascade loop controller was implemented. Inside this loop, an external controller computed the references to an internal controller which control laws were directly applied to the entire non-linear model. This work designs two different internal controllers based on the strategies mentioned above and relying on the dynamic model of the AUV. In addition, these controllers share the same external controller which is a non-linear PID relying on the cinematic model. In order to measure the performance of each controller, the results were quantified by using Error Indexes. Also, a robustness test was implemented. Advantages and disadvantages about using these controllers were briefly discussed. It revealed a better error correction when the robust controller was used.