A subsea measurement system (400) comprises a first reference beacon (110) disposed at a first known location, a second reference beacon (112) disposed at a second known location, and an acoustic communications module (302) for coupling to a subsea element to be monitored. The acoustic communications module (302) is capable of moving, when in use, relative to the first reference beacon (110) and the second reference beacon (112). The acoustic communications module (302) also comprises a processing resource and is arranged to determine first range-related data to the first reference beacon (110) in response to receipt of a first signal by the acoustic communications module (302) from the first reference beacon (110) and second range-related data to the second reference beacon (112) in response to receipt by the acoustic communication module (302) of a second signal from the second reference beacon (112).