Many active plate boundaries, such as mid-ocean ridges and trenches, are under the sea. Seafloor crustal deformation data will contribute significantly to understanding the nature of the tectonic processes at these plate boundaries. We have developed a seafloor positioning system with a GPS-acoustic link. This system consists of two main components; (1) the surface positioning by differential GPS to on-land reference and (2) the precise acoustic ranging using the M-sequence between the surface and seafloor references. The position and attitude of the surface GPS-acoustic link unit are determined from the GPS observations. Simultaneously, the acoustic ranging between the surface unit and seafloor references are carried out. The positions of the seafloor references are determined from these observations and a sound-speed structure model of the seawater. We performed preliminary seafloor positioning experiments. In these experiments, simple 1-D structure models are assumed for the sound-speed in the sea. The results show that the positions of the seafloor references are estimated with an accuracy on order of 10 cm. The residuals for acoustic ranging imply that there are systematic differences between the assumed and real sound-speed structure. It is necessary to reduce the uncertainties of sound-speed structures for more accurate positioning.