Mermaids

Your best friend, as strange as it sounded, was a mermaid.

She was a shockingly fun friend, despite the fact that she could only visit you in some place that has water. Sewers, pools, beaches, lakes, even your bathtub.
It was strange, and it took you for a long while to get used to it. Mermaids had an ability to transport themselves through the water, to any place they wanted. Their bodies were made up completely of water, water the solidified itself to shape their bodies. There was a special coloration factor to the water that allowed them to be different colors.
Most of the time, it depended on what they were feeling at the time. If they were sad, they were blue, if they were angry, they'd turn red, if they were happy they'd turn yellow. The colors would intensify as the emotions grew stronger.
Because of this strange body configuration, mermaids could travel between bodies of water. Through pipes and across oceans. It's pretty cool.

You met your friend when you'd gone to the beach when you were a kid, you'd been making a sandcastle and she had come out of the water to help you, giving you shells that you kept to this day. The second time you saw her was at the pool that was not far from your house. You didn't know how she got there at the time, but you were five and happy to have a friend.
A friend that never left.
From that point on, you were almost inseparable.

There were some concepts in mermaid culture still baffled you. They didn't celebrate Christmas, but they did celebrate Easter. But for them it wasn't a bunny that delivered the eggs, it was a Jellyfish. And though they had the Tooth Fairy, it was a Tooth Pufferfish. Small things changed and were tweaked.
There were only two seasons underwater, the warm-farming season and the cold less-farming season. There were several things you could farm when the water's cold, but it wasn't easy.

Aquatta, your mermaid friend, had spent a large amount of time trying to learn the human customs, just like you had hers. It had taken you a while to explain the her that human drown due to too much liquid consumption. You'd told her that humans couldn't breathe underwater and didn't generally have fins. She was tried to teach you to breathe underwater, saying that she was absolutely sure that you could if you tried hard enough, but that only led to you almost drowning in your own bathtub.
After that, she tended to leave to subject alone.
But that didn't mean that she wanted nothing to do with human culture. There were, of course, things that she didn't really understand, like television, telecommunication, barbecues, fire, forks, et cetera. She grew to love tacos, which you never really understood, she loved tennis because she said it looked fun and thought football was the funniest thing.

"Look at the way they run with their legs!"

But there was one thing, that always freaked her out. Drinking.
Drinking anything, not just alcohol. Water, juice, soda. She knew that it humans consumed to much water in their lungs, they'd drown.

"What are you doing?!" Aquatta yelled at you from where she was floating in the pool. It wasn't a big pool, just a kiddie pool you'd had since you were a kid.

"I'm having a drink." You replied to her.

"But you'll drown!"

You sighed, taking another big gulp of water. Unfortunately for you, it went down the wrong way and you began to cough.

"Oh my god! Oh my god! What's happening? Are you drowning? Are you okay? CanIHelpYou?" Her words jumbled between each other.

You coughed a little bit more, your face going red as Aquatta started to make strange bubble-like noises. The kind of noises you'd make if you blew bubbles underwater.

"I'm - I'm fine!" You coughed out.

She swam up to you and began hitting your back, which didn't really help, only making you cough more. She started splashing around, causing a bit more water to go into your mouth and you coughed more.

Yeah, most of the time you chose not to drink in front of your friend.