An aquatic time synchronisation system comprising: an aquatic first acoustic communications apparatus capable of communicating acoustically with a dynamic aquatic second acoustic communications apparatus, the aquatic first acoustic communications apparatus comprising a first time source and the dynamic aquatic second acoustic communications apparatus comprising a second time source; and an offset calculator; wherein the aquatic first acoustic communications apparatus is arranged to communicate a first acoustic signal having a first acoustic propagation path associated therewith to the dynamic aquatic second acoustic communications apparatus and the dynamic aquatic second acoustic communications apparatus is arranged to communicate a second acoustic signal having a second acoustic propagation path associated therewith to the aquatic first acoustic communications apparatus in reply to the first acoustic signal; characterised in that: the aquatic first acoustic communications apparatus is substantially static; the dynamic aquatic second acoustic communications apparatus is arranged to calculate a Doppler shift (Dop A2) observed thereby in respect of propagation of the first acoustic signal along the first acoustic propagation path; the aquatic first acoustic communications apparatus is arranged to calculate another Doppler shift (Dop B3) observed thereby in respect of propagation of the second acoustic signal along the second acoustic propagation path; and an average Doppler shift (Dop TB) is calculated using the calculated Doppler shift (Dop A2) and the another calculated Doppler shift (Dop B3), and the offset calculator is arranged to use data associated with the first and second acoustic signals (306,308) and the average Doppler shift (Dop TB) in order to determine a time offset between the first and second time sources (115,119). an aquatic first acoustic communications apparatus capable of communicating acoustically with a dynamic aquatic second acoustic communications apparatus, the aquatic first acoustic communications apparatus comprising a first time source and the dynamic aquatic second acoustic communications apparatus comprising a second time source; and an offset calculator; wherein the aquatic first acoustic communications apparatus is arranged to communicate a first acoustic signal having a first acoustic propagation path associated therewith to the dynamic aquatic second acoustic communications apparatus and the dynamic aquatic second acoustic communications apparatus is arranged to communicate a second acoustic signal having a second acoustic propagation path associated therewith to the aquatic first acoustic communications apparatus in reply to the first acoustic signal; characterised in that: the aquatic first acoustic communications apparatus is substantially static; the dynamic aquatic second acoustic communications apparatus is arranged to calculate a Doppler shift (Dop A2) observed thereby in respect of propagation of the first acoustic signal along the first acoustic propagation path; the aquatic first acoustic communications apparatus is arranged to calculate another Doppler shift (Dop B3) observed thereby in respect of propagation of the second acoustic signal along the second acoustic propagation path; and an average Doppler shift (Dop TB) is calculated using the calculated Doppler shift (Dop A2) and the another calculated Doppler shift (Dop B3), and the offset calculator is arranged to use data associated with the first and second acoustic signals (306,308) and the average Doppler shift (Dop TB) in order to determine a time offset between the first and second time sources (115,119). the aquatic first acoustic communications apparatus is substantially static; the dynamic aquatic second acoustic communications apparatus is arranged to calculate a Doppler shift (Dop A2) observed thereby in respect of propagation of the first acoustic signal along the first acoustic propagation path; the aquatic first acoustic communications apparatus is arranged to calculate another Doppler shift (Dop B3) observed thereby in respect of propagation of the second acoustic signal along the second acoustic propagation path; and an average Doppler shift (Dop TB) is calculated using the calculated Doppler shift (Dop A2) and the another calculated Doppler shift (Dop B3), and the offset calculator is arranged to use data associated with the first and second acoustic signals (306,308) and the average Doppler shift (Dop TB) in order to determine a time offset between the first and second time sources (115,119).