This paper presents the design and experimental implementation of an L1 adaptive control on a tethered underwater vehicle. This controller, well known for its fast adaptation and its robusntess, is proposed to be applied for the first time in the field of underwater vehicles control. This paper summarizes the implementation and experimental results obtained on a modified version of the AC-ROV underwater vehicle. Various experimental scenarios are presented to illustrate the ability of the L1adaptive law not only to successfully control pitch and depth (even with strong modeling uncertainties), but also to be e cient towards disturbances like waves or buoyancy changes.