Joss Whedon owns all things Buffy (including Dawn) and Disney owns Marvel comics and the Sub-mariner. I own neither and will make nothing from this. Even if someone were to offer me cash, I would turn it down because I'm honest like that...
AU a bit on the Mariner side...
"But I'm breathing underwater..."
She was standing on a street corner, watching the flow of a bustling city stream past her face. A blue man, like the performance artists but much larger and meaner looking, was being towed in a little yellow cart by two actual dolphins or porpoises or whatever. They slid by with a slight ripple in the water buffeting her where she stood a few steps away on the edge of the sidewalk. She didn't even raise her eyebrows this time because after only a few minutes of being here, she had seen much stranger things swim along the brick paved roadway.
"Excuse me! Does anyone know why I'm breathing underwater? And, why am I just standing here and not...floating or whatever?"
People, if you were charitable and wanted to classify them as such, crowded along the walkway, hurrying in both directions like the herd does in any normal city...except, you know, this one was underwater.
"Is this heaven? Like, you know, maybe you sprung a leak...or a big water main broke...that happens, right?"
Everyone was ignoring her. Once a while, the white and black striped guys with all the tentacles would pause close by and seem like they were scoping her out but as soon as she turned towards them, they hurried off.
"Hello! Umm..does anyone speak something other than...fish?"
Two big eyed obvious males wearing matching black armored chest-plates and looking remarkably like angel fish with legs and arms cautiously came towards her . Their language had a lot of bubbling sounds and she tried her best to pantomime that she didn't understand, wasn't from around here, and in fact, had no idea why she was able to breath. They of course took on the stance and attitude that adults do when they think they're speaking to idiot children and the same words they bubbled earlier were bubbled again but much slower and louder this time.
"I really don't understand," she said, also slowly and loudly.
The fish guys looked at each other with their massive round eyes and then began motioning to her that she should follow them. They were clearly treating her like she was a suicide jumper or something and she was starting to get a little frustrated.
"Look, I was just waiting in the shop for a friend to get off work so I could show him a box of comic books I found at a garage sale on the way home from school, then poof, I'm Aqua Girl."
They kept treating her like a skittish bunny rabbit so she decided to follow along and see where they were trying to get her to go. As their odd procession traveled down the sidewalk, blue people and fish people and all the others still paid the scene little to no attention. One of the tentacle guys stopped by and said something to the fish guys, who immediately whacked him in the side of the head and chased him away. She was trying to figure out a way to mime a question about that when the pair turned and pointed at what really looked like a police station. There were more black armored fish guys and even some armored blue people milling around the entrance and she was positive that this was the underwater version of a cop shop.
"Oh what the hell," she said to herself and followed them inside, wondering the whole time if non-fish or blue people showed up a lot because no one except her two friendly officers seemed to care that she didn't belong here.
They passed desks and even a holding cell where a very angry looking Charlie the Tuna guy was busy throwing up around himself in between banging on the cage walls. Her escorts motioned her to sit at an empty desk and gave great relieved sighs when she did so before they both hurried away, clearly eager to have nothing more to do with her.
After an hour, three different blue guys had come up and tried to talk with her. Each had looked at her oddly when she spoke back to them and they had all directed her to stay where she was before running off.
Finally, after who knows how long later, another older blue guy came in the front door and was shown to her. He sat in the desk and spoke something that sounded like it might be French, French spoken under water that is. Unfortunately she didn't speak French except for Bon Jour and Adieu, and he had no reaction to them.
She squeaked and nearly fell out of her chair before regaining her composure and smiling weakly at all the staring officers. "English, Yes! I speak English, yes I do."
"Thiss iss good," he sounded like she imagined a snake man would and her huge smile faltered a bit when he took out what really looked like a ticket book and started filling out one of the forms.
"Umm do you get like a lot of American girls here or something? No one seems to care that I can suddenly breath under water and by the way, why are we all walking around instead of swimming?"
The blue guy stopped writing and sat back in the chair, looking at her. "Iss thiss a game?" he asked.
"A game? How in the heck would I be playing a game where I can suddenly be Aqua Man's little sister?"
"The officerss that brought you in ssay they thought you might be a prosstitute and..."
She screeched and stood up so quickly that a small picture frame fell from the desk and broke on the floor. "A prostitute?! Are you kidding, I haven't even kissed a boy yet! ...or a girl for that matter, no discrimination from me," she finished quietly as she sat back down and apologized while he cleaned up the glass.
"Why don't you tell me what happened?" he asked as he retook his seat and wrote a few hurried sentences into the book.
"Okay, I was waiting in a store called the Magic Box for a friend so I could give him a box of comic books I... bought, with my own money, from a garage sale," she paused as he raised one of his hands.
"What iss comic book?"
"Just, umm books with like pictures instead of words. These were all older looking ones with stories about a submarine... Hey do you guys have submarines? Maybe they were magic comic books?"
"Ssubmarines? Not like you would imagine, no." He appeared to be quite confused by her and leaned back in his chair again.
"So DO you get a lot of American girls here?" What other reason could there be that she wasn't a spectacle to these people?
"Not common. Mosstly for the rich but it doess happen on occaision."
"How? I mean how can we be breathing and not freezing and the pressure and stuff?"
"Sorcories, magic, technology, there are ways if you have the money."
Dawn was about to ask just how a prostitute and those fish guys could possibly go together, when a soft voice sounded over her shoulder. "Excuse me, Officer. My... apprentice got a bit lost."
"Willow!" Dawn screeched, causing everyone to startle again.
"Young Lady, you have lots of 'splaining to do when we get home," The grinning redhead said.
"It wasn't my fault! They were magic comic books! ...And we're breathing under water! How cool is that?"
"Very cool, and Giles knows you went into his 'locked' cabinet. We found the Rytorrak Crystal of Summoning on top of your box of comics."
Dawn winced, wondering how she was going to talk her way out of this. She really was too young to be unsupervised in a magic store. This was clearly all their fault. "How mad are Giles and Buffy?
"Buffy will be fine when we get you home, but Giles is talking in tongues he's so mad and that's nothing compared to Anya."
"Oh...it's going to have to be a really good apology with huge puppy dog eyes, I think."
"I think a mop and a lot of elbow grease are going to work better for you. There's like five hundred gallons of saltwater all over the floor."