Des Newman at Oziexplorer had already solved the problem. I just had to think laterally (and have an allergic reaction to the wildfire smoke that made sleep impossible). Oziexplorer allows one to set up alarm zones using specialized tracks!
Now the picture can be re-imagined as this:
And one can build it and any other shape in Oziexplorer by using the track function and designating the track as an “alarm” track. Making this designation includes styling the area involved (hatching) so one can make sure the area was the one desired.
If the boat moves into the diagonally hatched area a cruiser-defined alarm will sound — to include .wav files of a voice (or you name it) instead of the annoying and ambiguous and ubiquitous 555 chip’s beeeeeeeeeeep. So now, for us, an Aussie lass bangs a gong and says, “Hey Mate! Check the boat position!” [Voice courtesy of Loquendo’s online TTS service. Voice choice was ours not Oziexplorer. The gong I had.]