It can be unlocked after completing Memo Task, and accessed either by interacting with the Starsea Voyager Vehicle, or by talking to Elsasani.
Overview
To head-out on a Starsea Voyage, Planet Tenders must have at least Lumia Battery ×1, which can be purchased from the Loomix Mart.
Planet Tenders can then select up to 2 Neighbors to assist them, with each Neighbor differing on which tasks they excel at, are good at, or are not strong at.
Upon setting out on the first Starsea Voyage, Planet Tenders will start in the Shallows of whichever Galaxy they selected (currently Hexia or Taffeh), with deeper Uncharted Bays becoming available upon unlocking the various Star Gates with Islet LucadropsIslet Lucadrops.
Other Galaxy's Starseas can be explored when departing from that particular Galaxy.
Islets
Islets are the main exploration content of Starsea Voyages. They are instanced zones, typically focusing on a specific Creature type, Item, or will feature a puzzle that can be completed for extra rewards.
Each Islet will initially be Unknown, notated with a " ? " icon, however upon exploring an Islet, its name and core type will be revealed, and can be returned to at a later date.
During a Starsea Voyage, Planet Tenders may also come across Encounter islets, which will be home to a Neighbor. These islets can spawn randomly upon starting a voyage, or during a voyage, and once the Neighbor has been greeted, and the Islet left, it will despawn and cannot be returned to.