In my own words, Neptunian culture is a difficult to relate to in literally any geographical fashion, but it's a culture emergent of likeminded beings/entities/souls/folks. In one sense it can be traced to a shared 'syncosm' on older postfurry MUCKs around the internet that i wasn't actually a part of~!
it's persisted over... literally generations now through folks like us who resonate with it and utilize it in ways that vary personally, and in seriousness.
but yes, in my words, if i'm to be authentic to that...
Neptunian culture is very aquatic. Whether Neptune (the ideal/utopia/root) or Halcyon (The planet out somewhere in the conceivable multisomewhere), it's ocean and sea. islands too, but the planet is mostly water. it's posthuman. technology and nature in harmony, in a way.
It helps to note we're very empathic/telepathic. Neptunians share this technology called the Mindlink, or Sea of Voices. it's a sort of direct-thought communication device that allows us to communicate empathically/telepathically through a serious of like... concepts or emotions. This definitely varies based on how far away someone is, and how close we are to someone.
At it's best though, it's intense, and it kinda informs a lot about every other aspect of our culture?

@Oneironott It's hilarious that Neptune is, tbph, the 12Fold I put the least effort into and thought I did the worst job on, and yet AFAIK, it's the only one to survive. (Except of course the Martian diaspora, which consists of... um... me.)