“Ian I can’t get down!” Kali yelled from the highest branch of the oak tree.
Comma between Ian and I.
“Yes you can. Just go down one branch at a time,” he replied.
Comma between Yes and you.
She didn’t dare look down, simply because she knew it would cause her to fall.
Try to find a less obvious reason. Maybe she's starting to get weak? She'd lose her grip?
Kali jumped off of Ian and as he stood up, she kissed him. Ian turned beet red and turned his glowing face to the ground.
This seems shocking, especially because it seems so out of character of Kali.
I tried to kick to the surface again, but the water was too deep now. I sucked in brackish water. My lungs burned and my heart slowed. A red-black cloud was forming at the edges of my vision. I swallowed more water and my body started to move in slow motion. I needed air. My eyes closed and I stopped moving entirely. I felt a last moment of peace before I gave into the sweet, caressing darkness.
She should be experiencing panic or fear, not peace. Sweet, caressing darkness doesn't exactly match the description. Is it crushing her, suffocating her, putting pressure on her? I also like the "red-black cloud" vision part, which fits the mood much better.
And what is she thinking? Is her life flashing before her eyes, is she seeing Ian on her mind?
My eyes closed and I stopped moving entirely.
Maybe you can go more into her muscle movement, make her completely aware of every body movement.
This is okay, starting out. It seems a bit cliche starting out. But post some more, clean up what you have, and maybe you've got the beginnings of a story on your hands.