So we all agree that Kwazii and Shellington are neurodivergent, right? Right. But hear me out: Captain Barnacles is autistic.
*cracks knuckles* let's goβ
His voice is very stern, monotonous, and very loud, at all times. He always speaks like this, even when he's being casual or funny. No matter what emotion he's feeling, either.
His catchphrases are part of his social script, the octo-alert is part of his routine, and he gets visibly bumbly when they're interrupted or triggered unexpectedly.
He very often does this thing where when the octo-alert is randomly sounded, he will instinctively react and begin saying his catchphrase, before getting really confused.
See also this scene where, really, there's no point of sounding the alert, as they're the only ones there, and everyone else is busyβwhen Tweak is unable to do it, Barnacles pulls out his compass, and does it himself. Plus the time when he sounded the alert telling everyone to go to the launch bay, but they were already there. He also has a morning routine, seen in ROF.
He always knows when Tweak has an invention that is perfect for whatever oddly specific situation they're in, when nobody else does. Likely because he doesn't like surprises/unexpected things, so she always tells him when she starts a new project.
Sensory issues! Observe his reaction to a loud noise:
Yes, everyone reacts negatively to the noise, that's true, but look at him! He completely recoils and tucks into himself, he tries to flatten his ears, he shuts his eyes, and he even shakes his head.
Special interests: music (his accordion), ships (his tiny hand-built models), and the octopod itself.
He wrote 3 books! 3!!! Just on how to pilot the Octopod. For who? Himself. Who else could actually read these books, and understand any of it? Even Peso in that scene says he doesn't think he'd understand it.
The octocompass is a comfort item for himβthe entire cuttlefish episode is him being stressed about getting it back after it's lost, and he gets easily frustrated whenever it doesn't work.
I'm out of space for pictures, but he struggles with social cues sometimes too.
Amazon River special: Kwazii has to directly say to him "that's your cue." when he misses his hint to sound the octo-alert.
Seahorse Tale ep: Kwazii is worried Barnacles is mad, and he doesn't notice at all.
Great Barrier Reef: Kwazii whispers the secret ingredient to him, and he repeats it out loud.