Logistics problems and high costs necessary to carry out experiments with robots in hazardous environments such as the open sea led to the development of simulation environments (SE) where the physical robot and the operating environment can be substituted by real-time simulators in a way that is completely transparent to the robot's control and sensing modules. SEs are extremely useful tools which allow to rapidly designing, developing, testing and evaluating in lab the working of an underwater vehicle and to minimise the risks to lose expensive equipment during in field testing. In this paper the SE developed by the National Research Council, Institute for Ship Automation of Genoa (CNR-IAN) will be presented.