Works completely offline, and has been completely accurate in the Pacific Northwest, where water moves in mysterious ways.
It really irritated the owner of the yacht, because it was so good, but unavailable for Apple.
…which is in turn a Windows version version of XTide. I suspect many such apps are built on top of this.
For those who want to run on something else (Linux, Mac, iOS, etc) there’s a listing of ports for different platforms on Available ports for unsupported platforms including one for the classic HP48G calculator. That’s almost enough to tempt me to break out the soldering iron and wire up a new data cable for it.