XTide is a package that provides tide and current predictions in a wide variety of formats. Graphs, text listings, and calendars can be generated, or a tide clock can be provided on your desktop.
XTide can work with X-windows, plain text terminals, or the web. This is accomplished with three separate programs: the interactive interface (xtide), the non-interactive or command-line interface (tide), and the web interface (xttpd).
The algorithm that XTide uses to predict tides is the one used by the National Ocean Service in the U.S. It is significantly more accurate than the simple tide clocks that can be bought in novelty stores. However, it takes more to predict tides accurately than just a spiffy algorithm—you also need some special data for each and every location for which you want to predict tides. XTide reads these data from harmonics files.
Ultimately, XTide's predictions can only be as good as the available harmonics data. It is up to you to verify that the predictions for your locale match up acceptably well with the officially sanctioned ones.
The XTide software distribution resides at https://flaterco.com/xtide/files.html.
XTide and its documentation are maintained by David Flater (dave@flaterco.com).