The purpose of this paper is to present Thetis: a real-time multi-vehicles hybrid simulator for heterogeneous vehicles. This simulator allows Hardware In Loop (HIL) simulations including virtual sensors which allow to provide a representation of a virtual world, and including the support of communication devices. The architecture of this simulator is conceived so that it ensures a temporal decoupling between the virtual environment, the vehicles, sensors and communication simulators and, of course, the actual embedded controller which warrants us the quality of the results. This paper presents a classification and a short state-of-the-art of the different types of existing simulators. Then we will introduce the architecture and functionalities of Thetis. Finally we will compare results obtained during sea-trial with our AUV Taipan300 and with our simulator.