Saltstraumen in Norway is the strongest tidal current in the world. Every six hours, hundreds of millions tons of water, brought in by the tide, rush through the narrow straight separating two fjords. When tides are very strong, water speed can exceed 30km/h, resulting in massive whirlpools and vortices - leaving a narrow zone of clean laminar flow in the middle. All this is quite a sight, but keep in mind that you need to time your visit at peak flow. Otherwise, it can be as exciting as watching water at a port ;)

More info about the place here