Remotely operated vehicles (ROV) and Autonomous underwater vehicles (AUV) are increasingly being used for a large spectrum of maritime tasks for which human intervention is either dangerous or impossible. In case of complex tasks such as detailed inspection, difficult environment or even budget shortage, AUV may not be suitable. On the other hand dealing with repetitive actions or having to constantly react to obstacles may be excessively tiring for the ROV operator. In this paper we describe a low-cost Laser-based obstacle sensor which combined with the camera image allow the ROV to autonomously react without the intervention of the operator and even blocking his commands. In order for the ROV to take control, a C++ software was developed to receive ROV sensors data, process it and send commands using the same serial communications port the operator uses. All the calculations related to the Laser sensor are performed by a Matlab program whose results are shared with the former program. Tests were carried out in a swimming pool and the semi-autonomous ROV performed satisfactorily and efficiently.