A data collection system 100 for estimating a corrected sound speed between a first point and a second point comprises a subsea acoustic transponder beacon (106) disposed at a fixed position relative to a seabed (108), and an acoustic transceiver (214) immersed below a sea surface (104) and above the acoustic beacon (106). A processing resource (200) is also operably coupled to the acoustic transponder (214), the acoustic transceiver (214) being arranged to traverse a path above the acoustic beacon (106) and communicate with the acoustic beacon (106). The processing resource (200) is also arranged to collect time-of-flight data associated with the communication between the acoustic transceiver (214) and the acoustic beacon (106).