AS someone who teaches open water navigation and coastal navigation, I agree that the term precision is an overstatement when concerned with open water, but when near known coasts or landmarks you could get relatively precision (enough to get the job done.)
That said, a map had to be implemented for game-play reasons. Otherwise, I'd either go look one up that someone painstakingly made or be forever lost, without ever really knowing where to go for anything. As for a sextant, how it'd be implemented without being overbearing is the key. As someone who's done SAR, a map without a compass or a reference point of some kind (whether that be landmarks, the sun, sky, or prevailing wind) is not particularly useful. While we do have a compass in travel mode (assuming it's adjusted to true north rather than magnetic north), and we can assume map orientation is always adjusted north as well, it's still a bit of a risk to sail directly across the Caribbean as is with the expectation that you'll arrive where you intend to arrive, instead of another random island, city, or (if you're rather unlucky/inattentive) way out pass the island chain that leads into the Atlantic.
With our current tools within game, we're put back much further chronologically in ship navigation technology and precision (for what it is) than what would be expected.
Now, I firmly believe we need tools of some sort. A sextant? Possibly, but possibly (and probably) not the best of solutions. Do I want a precise mark on the map wherever I'm at with GPS like precision? Hell no. But I feel there definitely needs to be some sort of middle ground.