So as the game has been out for a few weeks now, it is pretty fair to say the game is beautiful but quite shallow on the rewarding content that you need to give you a reason to keep sailing.
Rare released this video outlining their content plans.
6 larger content updates.
1 in May
2 in Summer
2 in Fall/Winter
All add a new AI threat.
Inbetween content is Weekly Events. These will utilise more of the map to explore things like underwater caverns and reefs, secret treasures and thrones to find with unique rewards.
All of this will be free (in fairness I agree it should as £50 for this game was extremely high) so by half year I feel this game should be extremely playable.
I can't say enough how beautiful the game is so with more content to help us along I'm relieved to say the least.