The High Seas is a Location in Divinity: Original Sin 2 which focuses on the ship called Lady Vengeance which is also used to reach Reaper's Coast. The High Seas is the next location after escaping Fort Joy. The two main quests that players will need to complete are Lady o' War and To the Hall of Echoes.
Once you leave Fort Joy and board Lady Vengance, your current team will be disbanded. Regardless of that, all your possible party members that are on board the ship can be re-invited. Just take note that after leaving the ship, you won't be able to change your team.
Lady Vengeance Sub-Areas Quick List
The High Seas: Lady Vengeance