Exllcent work

oh pretty

Water element!

what the hellicopter?

DOLPH-UNA!

WOW! She's just sooo beautiful!!!

The elder sister brings forth the sun each day, to create the shimmering ripples of light upon the surface of the waves, and give life to the ocean shallows,
While the younger sister brings forth the moon each night, raising the tides each night, and drawing forth life from the depths.

And that, dearest Twilight Sparkle, is how that old prophecy would have read if we were seaponies!

Beautifully well done.

critique:
hmm... those rings of water in the air look more like magic, but since her horn is not glowing i doubt is that.
you need to put a shadow on them, the highlights shouldnt be totaly white or that big, you need make the water rings more transparent, thinier and random (water doesnt stay totgether in the air that well) you need to give a more solid-shape too, right now it kinda looks like its evaporating at the edges
^all of that makes that your eyes pay more attention to the water rings than to luna, i didnt even noticed the "moon ears" or the actual awesomness of the mane.

-there are some parts where the water rings shouldnt be over luna`s body: the lock of hair below the neck, foremost fins, a bit on her wings, in the mane above the head and shoulders, attention details are what make a drawing beautiful.

if she is breaking the water surface, the water "splash" should look more or less like this: [link]...
but that would imply a ton of rework... instead you should try to darken the base of the splash and aply the same re-work of the water rings, to make it look more natural.

the clouds have the right coloring (highligh`s wise), its a nice style, though i would recomend that you experiment on then whit the blender tool/brush, to get a more real-like cloud, but thats up to you.

on luna`s boby...
the purple coloring in her tail looks its a nice touch, you should try to put it in the edges of her lower/fish- body as a light reflection of the night sky.
the shading near her cutie mark is confusing, is her fish-body suposed to be shiny, or hairy-like? it looks kinda erratic...
the shading on her uper-body needs to be softened or more expanded like it is on the fish-body
her hoofs, ears and horn needs to be shaded...

theres a line of darker blue on the mane above her face that its pointing upwards, that eyebrown-like line makes it look like she is frowning instead of looking peaceful, in the case it is the eyebrown, i gotta say its too long...

personal nitpicking:
(read as "me totaly critiquing your style")
-her chest seems a bit too square-ish, for my taste
-the shading makes her fins look way too thick than its naturaly normal on sea creatures,then again, this is fiction.
-that lock of hair in front of her ear is too daam dark, neads MOAR color! maybe a bit of normal blue.
-the base of her fins is too thick, it looks like is almost on her butt! though it would look weird if its too high, since its suposed to a fin not a whing...

sorry for the wall of critique...
its just that this is a great pic, and i think it could look way more greater