Based on some reading online, maritime delimination of boundaries beyond a 3 mile "cannonshot" distance didn't become common until after the 1950s when treaties

Based on some reading online, maritime delimination of boundaries beyond a 3 mile "cannonshot" distance didn't become common until after the 1950s when treaties were drawn up that would become the precursor to UNCLOS. In those treaties, the starting point for border delimitation was the equidistant point between two countries' coastline. When two countries have adjacent coasts, this ends up roughly perpendicular, but it still works even if they have irregular coastlines and doesn't involve any angle measurements, just distance. Considering that ocean borders only became common after the age of British colonialism and that perpendicular baselines were never specified as the standard in UNCLOS, I'm inclined to say that this is a myth.