This update introduces more realistic Sun, Moon, and other sky mechanics as well as treasure maps that visually hint at the locations of buried treasure.
Note: the new sky mechanics are covered in more detail in this announcement. Check that out if you are interested.
The sky has been updated with some new visual features. The positions of the Sun, Moon, and (newly added) planets in the sky now follow somewhat realistic models, as if the Salt planet is revolving around the Sun in a solar system. The moon has its own orbit around the planet and therefore will go through phase changes throughout the month.
A waning crescent moon and a planet in the sky just before sunrise.

Other planets are now visible at night and early morning. These appear like bright stars, and can be used in addition to the Sun and Moon as a target for your Sextant to determine your world location. They also have their own orbits and so their positions in the sky will change throughout the year accordingly.
You also have a chance of catching sight of a shooting star at night time. These are fairly uncommon unless the planet happens to be passing through some comet debris.
Some meteors falling over the ocean.

If you want to keep track of the progression of time throughout the Salt year, you can now purchase a Lunar calendar from the Stargazer, a new NPC, whose ship can rarely be found out at sea. Once you've obtained one, place it on your ship and interact with it to open it.
Viewing the lunar calendar in-game

Treasure maps can now be rarely discovered in various places throughout the world. If you find one, interact with it in your inventory to view the map. They hint at the location of a nearby buried treasure.
There are currently a total of 27 unique treasure maps to discover and more may be added in the future.
Viewing a treasure map in-game

The implementation of Co-op is progressing along very well. The logic to sync most of the elements of the world has been completed. We are now finishing up some of the last necessary features and will start doing some heavy internal testing. After we feel that it is stable enough, we'll release it on a Steam branch for more broad community testing and continue to polish it there for a bit. There is no ETA on this yet as it depends on the quantity and severity of problems we find as we stress test it.
In parallel to co-op work, additional content, fixes, and polish will still continue.