In order to upload a vessel you must first create and save one in game.

To upload a vessel that you created in game you must access the Steam workshop wizards located in the settings in the main menu (gear button, top right)
These wizards will allow you to choose various information required for easy upload of your vessel or mod to the workshop. Mods are covered in detail in a separate guide, but for vessels you simply need to select the desired vessel and follow the prompts. For the initial upload you can use the upload buttons. In order to update an existing workshop item you must use the update buttons on the right instead of upload.

Once uploaded, you can use the Steam workshop to edit any desired fields or add more screenshots.

Note: The default preview image will be generated when you save a vessel, it will be named "[VesselName]_ForWorkshop.jpg", you can manually overwrite this file prior to upload / update if you want a better preview image, but it will be overwritten the next time you save the source vessel. Options may be added to directly add this in the future. You can located these preview images in the corresponding folder here:

C:\Users\(Username)\AppData\LocalLow\Revmatek\NautikinAdventures\Saves\(SaveName)\Vessels

In order to subscribe to a vessel you must use the Steam workshop and click "Subscribe" on the vessel you are interested in. Subscribed vessels will be loaded when you load a save and will show in yellow when you open the list of saved vessels.

To subscribe to someone's mod, simply click "Subscribe" in the Steam workshop page for the mod and the next time you load a save the game will load it in.

Some special notes about how that works for servers. Servers hosting a game with mods loaded will automatically trigger clients to download those mods temporarily in order to join the game without missing parts. The only exception to this right now is subscribed vessels. Vessels that have mod parts in them will not automatically download said parts. So any mod parts you use in your vessel you need to list on your vessel workshop page I guess. This may be supported automatically in the future.

There are a couple common reasons why an upload may fail:

• If you need to subscribe to the Steam Workshop agreements for your account. Sometimes they can fail once and work on retry, depends on connectivity to the workshop servers.
  
• If an image such as the preview image is too large or not in the correct format.
  
• If your Steam Cloud quota for the game is full (workshop files seem to be stored in the same space as your game save files, so if you have lots of save files this may become an issue).

If the failure is unclear, you may be able to lookup the error code in the log file.