You can know longitude from the stars. Pole Star shows it
"My suggestion: On good weather conditions take the position of the ship at 12:00 o'clock and mark it on the map."
I think we need a 'Capt' log' or 'ship's logbook'. A captain need to take notes (auto, notes on the map). Marked the battles, the route, the seen ships. Maybe, showable marks on Open World could useable to navigate.
Plus: spyglas why only available in the battles? Not really useable.