Living an ocean-world, Neptunians naturally have a talent for swimming, with the stamina and speed to get them to most places they need to go. That said, there are occasions where self-power might be insufficient, such as navigating the streets of a dome-city. Thankfully, Neptunian technology is accommodating to those needs.
In general, movement devices are personal-scale, for one or two riders. A large reason for this comes from their tradition of individuality and personal style: A jetbike is as much an accessory an extension of who they are as it is a tool. Almost always, they're designed by their rider. That isn't to say that there are bog-standard ones, they are just far from common: the most frequent example being an off-worlder using a rental.
The other reason for this is the Neptunian's affinity for psionic connections. Typically, when one fabricates vehicle fabricated, they go to a local technician, that fits it with an AI. The vehicle's intelligence is set to communicate exclusively to it's creator's wavelength, and as the rider uses its craft, that AI learns and develops its own personality to match, essentially becoming a partner to the rider. Someone who isn't bound to a vehicle can still pilot it, but except in rare circumstances, doing such is considered a pretty harsh faux pas (and, expectedly, said craft won't function as well).
This psi-bonding can be performed on animals, the most common example being the Pelagic Corps of the Lifeguardians with their characteristic riding mantas. In those cases, there is a implicit degree of consent between the rider and the mount: the psi-bonding comes out of a degree of trust and friendship between the two. As such, bonded mounts are incredibly loyal and affectionate to their riders.
It is also said that, on rare circumstances, a living creature's intelligence can be extracted, and transferred to a craft to serve as a living intelligence- supposedly even more capable of handling a machine than an AI...But you'd be hard-pressed to get any details from a native.
Due to the Neptunian propensity for mind-blindness, it is somewhat uncommon for them to want to go visit other worlds. That said, it's also a bit of a challenge for other ships to land on Halcyon due to lack of actual land- so the more sociable few run trade and transit vessels throughout the system.
While most on-world vehicles resemble fish or common marine life, celestial vehicles often look to the uncommon. Tentacles are a common feature, tending to mimic squid or jellyfish; often times they look as if they may be bearing growths of kelp or lichen along their sides.
Neptunians don’t really believe in armies, or any sort of military spacecraft. That’s silly. Why would they want to fight anyone? Maybe, like, a rad sound-and-light system...