Abstract-The paper characterizes the performance of Autonomous Underwater Vehicle (AUV) localization when the AUV moves in environments where floating drifters or surface vessels are present and can be used for relative localization. In particular, we study how localization performance is affected by parameters e.g. surface object density, their visibility range and their motion. We present a discrete-time nonlinear model for an AUV equipped with onboard sensors and relative positioning information from surface objects (e.g., ranging and bearing) and derive the associated relative Posterior Cramér-Rao Lower Bound. We introduce a probabilistic motion model for the AUV, based on a random direction mobility model, to analyze the expected performance of the localization algorithm in terms of hitting time between the AUV and the surface objects. Finally, an extensive simulation analysis is performed using a discrete time Extended Kalman Filter with Maximum-Likelihood Data Association. As a proof of concept, an AUV equipped with an upward looking sonar is shown to detect a surface vessel and improve its localization estimate.