This paper describes a speed warning system for boats moving on a wide water surface using a single grayscale camera. Boats are detected, their speed estimated, and statistics are also computed. The approach is based on an enhanced background subtraction technique combined with classification and tracking to improve robustness; it provides a stable detection even with sea waves and strong light reflections and it is robust to tides which in Venice can reach high variations. The method returns correct speed values within the range 65% in the 97% of use cases. This system has been in operation for 3 years on the Burano island in Venice, monitoring a 250 m wide channel slice. The estimated speed is shown to the boats thanks to a digital display. Processing results can also be accessed remotely for monitoring purposes via web and even on mobile devices. Another system has been installed in Venice at the end of 2009 while a third is operative in Murano since early 2010.