The Beach at Eighteen

[dukealien]

Aqua breath cradles me. Tides of air soothing on sun washed skin.  "Aqua" is odd here - color, not scent of salt.  Simplify:  "soothe my sun-washed"
I am eighteen and she is beautiful.  good leitmotif here
She holds my eyes in a way
that makes my cheeks flush. maybe burn, sun and sensation?

Her rhythmic exhales comforting. you mean exhalations, which won't fit.  "exhales" isn't a noun, though
I have to hug my knees to my pulse. Seated here,
at the edge of her fingers. nice description, perhaps no period to get "fingers tapping"  - see below on punctuation
Tapping and crawling until my feet are wet.  "crawling" is odd... teasing?

I am eighteen and she is playful. 
Winking light along her surface.  blinking, but "winking" serves well 
Coy, patient goddess. She can hear my panting up against my legs.  a little confusing - as it should be
 I am excited to be afraid. a good line

To dance in her. Breath catching   good line break
in my throat
when she pulls at my waist. 
Love me. It was not a request.   colon instead of period - maybe even an exclamation point, this is a command!
Could mortal resist divinity.   question mark

I am eighteen and she is passionate. 
Yanking at my body when she kisses up my throat.  good, original - maybe "as"
Our chests heaving together. 
A gasp. Swept underneath. Only to burst,
from her, soaked in her. 
Eyes to the dying sun. good introduction of death factor

The horizon is on fire. My lungs draw it in.  and burn with it
I am eighteen and she embraces me. 
Life has no weight.  good, better than "life floats" or "is weightless"
Her world, a languid amber. Time unwound   excellent - trapped like a fly in amber
and dizzying. 
My axis twisted slowly. 
My world dissolved in her. tempting to say "hers" but this is better.

Her soft arms yank at my body. Deeper,  "yank" is not the word, "pull" too common.  You can do better, it's an important word here
the realization, and I am afraid to be excited.  the inversion here is unnecessarily clumsy (unless that's the intent)
Whirling, dance of spheres. Thrashing in terrible beauty.  "spheres" - worlds or bubbles, nice
My lungs emptying. 
I am eighteen and she is ravenous.   could end here - nice place for a page break, suspense

Do I rise, or, does she push me. I will never know.   question mark
The lesson learned when I crest and wheeze. "crest" is good here
Newborn babe. Taste of salt and needful fire.   spank - breathe!
Her world is not my world. I do not control here. 
I can lay back upon her bed.   lie, not "lay"
Lips drinking in my air. Resting on her palms. "Lips drinking in my air" is confusing - "drinking my own air?"

Wits return with peace. She leads this dance. 
I am eighteen and she is gentle. 
Forgiving of my fumbling, excited movements.  heh
Respect in rapture. Touching only how she wishes to be touched.   perhaps "as" instead of "how"
The rewards. 
My saltwater joins hers on my cheek. salt tears - ok

Her fingers coax me away when she is finished with me.  perhaps "brush" instead of "coax"
When my muscles burn with my skin. got a bit of sunburn, have we?
Returning me to the beach. 
My exhausted body 
laying under the dark sky.  lying, not "laying"

Hair sticking to my face. 
Palms sinking in the sand. 
Just watching. 
Breathing. 
I am eighteen and she is forever.   and the lesson at the end - well done

In intensive critique, a bit detailed but the work can stand it.

The style of line/sentences is suitably breathless and finesses the issue of line capitalization (they're all sentences).  But they are *not* sentences. being mostly sentence fragments.  It works, but having established that style you can also work against it where appropriate - where it flows, use commas or just join the fragments into sentences with a verb and all the trimmings (including a question mark or exclamation point).

For example, "My axis twisted slowly."  In the style you're using, this seems meant to be read, "My axis, (which is) twisted slowly" without a verb.  But a reader may get, "My axis (then) twisted slowly," which is present tense instead of sentence fragment.  Should you insert the comma?  Depends on what you want.

Overall, this is an effective narrative, tracking with closeups (still frames, at that).  The feelings come through.  As you edit, you might keep in the back of your mind how this would work written in third person ("He is eighteen, and she is passionate.")