Assassin's Creed: Odyssey has a big world map. You can use the ship to traverse it, which is easy to forget once you leave the ship somewhere. This problem is particularly apparent during exploration of bigger areas of the game. This section will let you learn how to find the anchored or moored ship and how do summon Adrestia if you fail to find it.
The easiest way to locate the ship is to search the map looking for the golden icon of the Adrestia ship (see above). The ship should by default be anchored in one of the ports and be visibly marked even if you zoom out all the way.
If despite all that, you cannot seem to find Adrestia, you should visit the nearest Ship Dock in the game. These can be found in every bigger city and village.
After reaching the dock, stand in the highlighted area and hold the button responsible for summoning Adrestia. She will reach the port without any further intervention. This will allow you to embark the ship and continue your sea voyages.