Abstract: Method for determining the location and/or speed of at least one swimmer, swimming through water, comprising: —sending acoustic signals through the water, using stationary acoustic transmitter means (AT) having a stationary position, or using mobile acoustic transmitter means (AT?) being carried by said swimmer (S); —receiving the acoustic signals using mobile acoustic receiver means (AR) being carried by said swimmer (S), or using stationary acoustic receiver means (AR?) respectively; —determining the time difference between the time of sending and the time of receiving each acoustic signal; —determining a location and/or speed of said swimmer (S) with respect to said stationary transmitter means (AT) or stationary receiver means (AR?), using said time differences and a predetermined propagation speed of the acoustic signal through the water.