I personally enjoy that there is no ownship position marker on the map to tell you where you are. It forces you to get acquainted with your surroundings and be able to visually identify where you are based on the coastline. Of course, nobody is going to have an intimate knowledge of every island and inlet, so that adds an adventurous element to the game.
However, while cartography and navigation was indeed often just a matter of mariners drawing up their own charts, there was also a fair amount of sophisticated instruments and practices that allowed vessels to navigate with efficient accuracy. I support the idea of adding a consumable you can use at your own discretion. For instance, each consumable is a "taking of one's position." It's accuracy depends on both the quality of the consumable and the time of day/visibility. If you try to use it in the dark, it won't do you much good.
When you take your position, a plotting mark will appear on the chart. This mark will remain at the exact spot you occupied upon taking the fix, so it won't tell you where your ship is at that exact moment, but rather tell you where you were so you can get a feel for where you are heading relative to where you want to go. After a certain amount of time (let's say a minute), the plotted mark will disappear.
I also think the idea of being able to use a pencil/ruler style of tool to draw lines would be nice. You could plot out the legs of your voyage and then take fixes to see how far off the mark you are. While this might seem to take away from the overall adventure of not knowing 100% where you are, recall that the fixes taken aren't all too accurate, especially if getting a quality consumable costs a shiny penny (accounting for the cost of the instrument and skill needed for accuracy). Considering how often we see fog, rain, etc. in the open world, it wouldn't be too hard to find yourself in a situation where you ventured to sea and are no longer confident as to your whereabouts. You then pray for a clear day to take a fix and determine where you are, but you only have two consumables left...