I understand how modern day ships can find each other with things like radar, but back in the 17-1800s, how did boats find each other in the middle of places like the Atlantic Ocean or the Caribbean sea?
With lookouts. A fair sized ship in the age of sail is visible a long ways off when under sail. Added to which, winds and currents dictate optimal routes for sailing ships to follow and these would have been well known. That meant that if you were looking for ships going from point A to point B you can pretty much deduce which route it will take and quite simply wait for it.