With the increase in the number of offshore power generation facilities and aquaculture ponds, demand for remotely operated vehicles (ROVs) to inspect underwater structures and conduct ecological surveys is increasing. Because ROVs are connected to a ground station on the water by a tether cable to enable the transmission of operating commands and images underwater, where radio communication is difficult, they can only operate within the range of the tether cable. To expand the operational range of ROVs, an autonomous surface vehicle (ASV)–ROV cooperative system, in which a tether cable is connected to an ASV and radio waves are used for remote operation from the autonomous vessel, is attracting attention. In a cooperative system, the tether cable must be managed on the ASV side, and the position and cable length of the ASV must be controlled to prevent the cable from pulling against or tangling with the ROV, which moves around underwater under the command of a human controller. We propose a method of controlling the position and cable length of the ASV using the information on the position of the ROV obtained from an underwater positioning device for an ASV–ROV cooperative system that we are also developing. In addition, we confirmed through simulations that the ASV and cable length follow target values even when the ROV moves along a complicated path.