Traditionally, currents are measured with buoys and drifters but these surveys are expensive and results are scarce. Almost all ships are equipped with AIS (Automatic Identification System), primarily to improve shipping safety. Broadcasted AIS signals contain information about the ship’s identity, position, speed and heading. These signals are collected at coastal stations where time stamps are added. The resulting time series make it possible to track individual ships and analyse their movements. This article shows the results of a case study in the Strait of Gibraltar making use of freely available AIS data which the presented method turns passing ships into moving buoys to obtain tidal information on surface currents.