Eras II is the most realistic game I know. The developers have done a pretty good job, but no mod and no game is 100% simulation.
As for the sandbox mode, it is pure open world, with total freedom to do what you want. You also have total freedom to navigate in real time on a giant map, (obviously on a reduced scale). You could spend hours traveling at normal speed from one end of the map to the other. You can also teleport to the nearby places and you can also speed up time and navigate quickly through the strategic map. In other words, there are two types of navigation, one based on total simulation and the other, simply navigation in a simplified way. Both options are well represented in the game. You can travel with the tools of the time and remove all kinds of aids. You decide which one to use at all times.
You also have story missions and repeatable missions. Eras II is not a linear game. You decide everything, but be careful, it is not advisable to accept a mission and fail, since your reputation is at stake. By the way, the system of reputation, title, morale and rank is quite interesting and addictive. Your character and most AI characters have their own reputation. If you commit acts considered morally good or bad, you will gain and lose reputation and therefore the sympathy of the npc who know you. You decide to be the hero or the villain. In either case, the progression of morale and reputation is slow, and good things are quickly forgotten over time.
As for the naval battles, they are quite spectacular and realistic. Of course you have to take into account the direction and speed of the wind. Properly use different ammunition for different purposes. You know, sail shots to slow down the opponent, shrapnel shots to cause crew casualties, normal cannon balls and explosive shots to break and set the hull on fire. The movement of the sea depends on the weather and storms, which makes it difficult to aim the cannons.
If you choose a novice character, at first it is more difficult to navigate and fight, because you do not have a suitable ship, no skills, no officers, and no proper instruments. In that sense, it is very addictive to try to get little by little the necessary equipment to simplify things.
There are many items and goods, some of them necessary to micromanage your ships. You need to buy wood and cloth to repair the hull and sails. Depending on your strategy, you decide which shipment of supplies to buy (ammunition, food, drink...). Your ships and your crew need logistics that are gradually consumed. A ship with a large hold can be useful for transporting goods, but speed is also very important.
Directional orders are used to make allied ships avoid the straight line. This option is very important when you see that your allied ship is in danger of being boarded. For example, in the original game, if you gave the order to your allied ships to flee, they would risk going in the opposite direction. Now with this new option you can prevent absurd dangerous situations for your allied ships.
Eras II has a rich set of mods built in that add realism. Any mod can be activated and deactivated at your choice in the options menu.
You can have a fleet of 6 ships maximum, but I don't recommend having more than 4 ships, in order to capture two ships in each battle. The larger your fleet and the level of your ships, the more money you need for maintenance. You know, it's always good to have one or two heavy ships, a cargo ship and a fast ship to break the opponent's sails. But ships are worth nothing if they don't have a good commanding officer.
I hope I have clarified some of your doubts.