The growing interest in generating electrical power from tidal currents using tidal turbine generators raises a number of environmental concerns, including the risk that cetaceans might be injured or killed through c ollision with rotating turbine blades. To understand this risk we need better information on how cetaceans use tidal rapid habitats and in particular their underwater movements and dive behaviour. Porpoises, which are the most abundant small cetacean at most European tidal sites and we have developed an approach which uses time of arrival differences of narrow band high frequency (NBHF) porpoise clicks at hydrophones in an array drifting in tidal rapids, to accurately track their fine scale movements un derwater . Extensive ground-truthing and calibration trials have been carried out that show that the system can provide depth and location data with sub meter errors and also indicate array configurations likely to provide the best balance of accuracy and p racticality. Field data from porpoises apparently foraging in strong tidal current areas reveal contrasting behaviours at different locations. A recent surprising observation has been of porpoises diving to ~100m in the Corryvreckan/ Great Race.