A component of the multi-purpose moored buoy has been improved for the instantaneous detection of seafloor displacement and possible tsunami generation upon the occurrence of large earthquakes. Here, improvements to the acoustic ranging component of this buoy, which is a key element of on-demand GPS/acoustic (GPS/A) measurements, are demonstrated. A 1-m positioning accuracy is required in GPS/A measurements using the buoy system for the detection of the horizontal seafloor crustal deformation associated with large earthquakes. Owing to the limitation of collecting data on only a limited range of sub-surface depth from a single point located far from the optimal location, resulting from the slack mooring system, obtaining the positioning accuracy is challenging. To overcome this challenge, we developed an automatic travel-time algorithm that reliably excludes the multipath from acoustic waves. Applying the algorithm to the data from 4 months of sea trial, we revealed that the short-period repeatability of the positioning improved from 4 to 0.5 m, while the long-term repeatability improved from 8 to 4 m, which is still beyond the required accuracy of 1 m. Because acoustic ranging under sub-optimal conditions will propagate any error in the data at intermediate steps into the uncertainty of the final positioning, approaches to reduce the errors at each step must be undertaken, such as determination of the pre-defined geometry of seafloor transponders more precisely.