Those who assume that because the game is created by Bethesda soft works (the same people who made Morrowind) and that therefore the game is as free as the wind will be disappointed. Although you can travel between islands at any time, and complete many sub-quests in between the main story, the game feels intolerably constricting. The majority of sub-quests for example are mostly simple “take cargo here” and “take passenger here” missions, with unique ones much harder to find (and often relatively short-lived when you do find them).
Compare this title to Morrowind, where you spent more time doing the sub-quests than engaging in the story, and you feel like your in some kind of old fashioned console RPG of following A to B to C to D religiously.
Even the missions themselves follow a set formula. One mission for example is to free a kidnapped son by paying the ransom off. However, I thought I’d be smart to look for where they were holding him captive and rescue him, rather than go to the arranged meeting place and pay them. I found the house and killed the sole guard, yet nothing happened. The captive was scared of me and wouldn’t talk to me! The ONLY way of doing that mission is to pay the captors… yawn.