GPS/Acoustic seafloor positioning has become an indispensable geodetic observation for the monitoring of crustal activities in subduction zones. There remain, however, some key problems to be settled. Among them is long-term stability of acoustic benchmarks deployed on the seafloor: long-term attitude stability, especially against ground motions of earthquakes, and their electrical/mechanical durability for long-term geodetic observation. M7-class earthquakes occurred in 2004 off Kii Peninsula, Central Japan, and then coseismic seafloor crustal movements were detected by using the GPS/Acoustic (GPS/A) observations. This event gave us a unique opportunity to test the stability. We carried out diving surveys in 2006 and visually inspected the benchmarks that detected the crustal movements. All of them stood stably on the flat sediment. No effects of the earthquakes were observed. In case that the slight tilts of the instruments were caused by the earthquakes, the effect on the observed crustal movements is estimated to be within 1 cm. One of old benchmarks deployed 6 years before was recovered and inspected. There was no problem on the battery, and no damages on the outer frame of the instrument. The diving surveys have settled a basic problem in seafloor geodesy and paved the way for long-term geodetic monitoring on the seafloor.