Young Writers Society

I closed my eyes, silently begging [Maybe hoping would be better here?] to wake up from the nightmare I assumed I was having. I was still struggling, but Lena was strong. I, on the other hand, was nothing more than a twig in a purple bathing suit. She tightened her grip around my wrist, and my stomach started doing jumping jacks. The water all of a sudden got significantly warmer, as Lena loosened up, just enough for me to slip out. [I think this would sound better if you said as Lena released my arm rather than 'as Lena loosened up...']
Then I looked around, and I couldn't swim away. I wasn't in Lake Kilter anymore. I looked around shocked. [You've repeated looked around here. Maybe use 'I stared, startled at the beauty around me.'] The murky, ice-cold waters of Lake Kilter had turned into warm, lagoon blue water. [Repetition of water here. How about murky, ice-cold currents of Lake Kilter. Fish that you would never see, not in Remove the in here. It isn't necessary. anywhere but here. You could look in every book about tropical fish, and never come across anything quite like these.
Fat blue fish with metallic red fins hid in the strange silver seaweed with flecks of pink, purple and orange standing out vibrantly on the shiny plant.
Something that I assumed looked like skinny gold dolphins swirled around, playing games at the surface of this bizarre water world.
Off in the distance, I saw a city, the buildings made of shiny rainbow colored rocks. Lena and I floated there in silence, until she said "This is where you are meant to live. This is your true home, welcome to Mare Nostrum Domus, Sea, Our Home."
I broke into a panic, making a mad dash to the surface. I wasn't going to live here! I tore up to the surface, gasping as my head pierced through the water. I took a breath of the fragrant air as I floated there. The blue water was like glass, and that was it. Blue glass, all around me. Going on, and on. It gave me a dizzy feeling. Lena swam up to me, a truly sad look in her eyes.
"I'm sorry that I had to bring you here, away from the lake. It was an order. I'm sure you'll get to go back soon. But in the meanwhile, would you like to go into the city?" She asked, shaking her short black hair, sending tiny water droplets flying everywhere.
I took in a breath, trying to be as brave, confident, and calm as possible. Beautiful birds flew through the air, similar to sea gulls, but in bright, lovely colors. It seemed like Mare Nostrum Domus was in technicolor.
Off topic I asked, "What kind of birds are those? And the fish, the plants? They're... beautiful!"
Lena looked up into the bright blue sky, like she'd never noticed the birds before. "Oh, well those are Wipples, truly lovely birds, though they never seem to shut up,"
I looked back up at them, wondering why a city with a Latin name would have such for lack of a better word, "cutesy" names for the wildlife.
As if reading my thoughts, Lena explained, "We're a truly ancient city, our original king was a wise man, knowing almost every language, even a few he invented on his own. His daughter, the princess, was given the job of naming everything," Then she continued in a whisper, "You can't say anything horrid about the titles though, the Royal Family doesn't approve of slandering their relatives."
I nod. For someone so young, she knew so much. I, being maybe a year or two younger than her, knew nothing about the presidents, or American History, or anything about my country. "Interesting. You're quite smart, I'm thirteen, how old are you?"
She sighed, letting out a stream of air bubbles. "I'm fifteen, now let's stop the small talk and get to the city. You'll need some new clothes, and you're hungry I bet? Well who cares, I sure am!"
I followed Lena nervously towards the lovely buildings, as she pointed out the species of fish and plants. I supposed it was quite lovely here, and I wouldn't mind staying for a while.

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Other than those pointers it's imaginative and I like how the plot has advanced.