During collision avoidance, the tether of Remote Operated Vehicle (ROV) is subject to entanglement with obstacles or other ROVs' tether. This specificity means that the classic literature on multi-robot obstacles avoidance is not suitable to tethered multi-robot scenarios. This paper proposes a guarantee ellipsoid model of the ROV's tether and its obstacles to perform a collision avoidance method for a fleet of ROVs, low in calculation. This model guarantees that if an obstacle is not inside or partially inside the ellipsoid model, then we are sure that the tether is not collide with that obstacle. This model is simple and can provide a guaranteed proof of non-collision without any knowledge of the tether shape or its dynamic, only its two attachments point and its length. A collision avoidance strategy has been developed based on potential field methods, tether's length management, and bypass path. When several ROVs are involved, personalities are added to ROV to obtain different behaviors in the same configuration and so limit the case of minimal local during collision avoidance. Simulations show the effectiveness of the method with several scenarios and the limits of the method are discussed.