The Darkened Sea Information & Guides:
There are two open zones and six locked zones in The Darkened Sea.
Progression is strictly linear, with a storyline (quest overview) that begins in Tier 1 and ends in the final zone. You must complete all partisan tasks in the order listed here. Missions, if you have someone else to request them, can be completed any time, but to request them, you'll need to have completed all prior progression tasks.
Basically, to access a given zone, you need to have completed all bolded tasks listed before it.
Requirements for requesting mercenary tasks and heroic adventures vary by zone. Some have pre-requisites; some don't.
Listed here are all tasks and group missions.