A communication system comprising a first underwater wearable device provided with a first communication module and a second underwater wearable device provided with a second communication module, wherein the first communication module at least comprises: - a signal source configured to provide an input signal (S 11, S 11b) to be transmitted; - an acoustic transmitter configured to convert the input signal (S 11) into an acoustic transmission signal (A T10) suitable for transmission in water and to transmit the same; wherein the second communication module, at least comprises: - an acoustic receiver configured to receive an external acoustic signal (A R10b) and to convert the received external acoustic signal (A R10b) into a processable signal (S 15b); - a signal processor configured to process the processable signal (S 15b); wherein the underwater wearable device with the first communication module and the underwater wearable device with the second communication module (10b, 10e) are configured to be worn at mutually distant locations of the swimmer, wherein the communication system is configured to derive information pertaining to the swimmer from properties of the acoustic signal transmitted from the first communication module to the second communication module, characterized in that the communication system is configured to derive information pertaining to the swimmer from properties of a version of the acoustic signal due to the behavior of the acoustic signal in water. - a signal source configured to provide an input signal (S 11, S 11b) to be transmitted; - an acoustic transmitter configured to convert the input signal (S 11) into an acoustic transmission signal (A T10) suitable for transmission in water and to transmit the same; wherein the second communication module, at least comprises: - an acoustic receiver configured to receive an external acoustic signal (A R10b) and to convert the received external acoustic signal (A R10b) into a processable signal (S 15b); - a signal processor configured to process the processable signal (S 15b); wherein the underwater wearable device with the first communication module and the underwater wearable device with the second communication module (10b, 10e) are configured to be worn at mutually distant locations of the swimmer, - an acoustic receiver configured to receive an external acoustic signal (A R10b) and to convert the received external acoustic signal (A R10b) into a processable signal (S 15b); - a signal processor configured to process the processable signal (S 15b); wherein the underwater wearable device with the first communication module and the underwater wearable device with the second communication module (10b, 10e) are configured to be worn at mutually distant locations of the swimmer, wherein the communication system is configured to derive information pertaining to the swimmer from properties of the acoustic signal transmitted from the first communication module to the second communication module, characterized in that the communication system is configured to derive information pertaining to the swimmer from properties of a version of the acoustic signal due to the behavior of the acoustic signal in water.