In Greedfall, there’s a lot of time spent on the mysterious island that will last you hours and hours of gameplay. The story, however, starts in Serene, a huge city where you’ll complete your first quests (including side missions) before setting sail.
There’s a lot of raw materials to be found in the city, plus some weapons and armour, characters and interesting stories.
When in the city the main things you’ll be able to do is follow the main quest. If you complete it though, it will trigger leaving the city.
So make sure to concentrate on your quest log and finish side missions and exploration before doing so.
Complete all the side missions before leaving Serene:
Make sure you also look around as there are plenty of chests hidden around Serene, including ones locked in rooms following side quests. If you need some additional lockpicking skills – Kurt your first companion has armour that gives him +1 lockpicking.
Sadly at this moment, you can’t.
Once you have completed the main mission, it will trigger your journey on the boat and won’t be able to come back.