This story starts with a boy, a boy holding his breath and opening his eyes.
Everything around him was rhythm: the movements of the wide-bladed grasses, the shifting of gold sand across the smooth black rocks, the ginger colored water rocking swirls of blue-eyed fishes. And all was just out of the reach of the boy’s small fingers.
The child, Amungstus played in the waves of the cold lake. On the shore servants unpacked wicker baskets, his father helped Amungstus’ brother onto a dappled wood-land pony and his mother lay under a tree reading a slim volume of poetry.
Amungstus dove under the water. He watched the fishes: eyes sparkling and flashing, pink-skinned bodies flipping, sliding thru the leathery marsh grass- swaying in a dark hypnotic rhythm, the oldest and deepest magic : inward, outward, shells, sand and souls.
Then the smallest fish, a speck of a minnow, broke from the school. The minnow was gray like the pallor of death and his eyes and body reflected no light.

Several other minor quibbles:

A child playing in a cold lake? For about 5 minutes before he turns blue?
If the minnow did not reflect any light, then it was invisible, wasn't it? Or was it glowing with pale dead gray light of its own?

The only question I had was from this line...

1. First line. The repetition of ...a boy... isn't necessary. The punctuation is troublesome here. I would think it should be either an elipses or a new sentence. Could be wrong. But it would be just as effective if you simply cut the second boy. A nicer line that way IMO. "This story starts with a boy holding his breath and opening his eyes."
2. Fourth line, third word. It should be set apart by a comma on both sides OR no commas, depending mostly on how you want the line to flow rhythmically. "The child, Amungstus, played..." OR "The child Amungstus played..."
3. Fifth line. Since this sentence lists more than two things that are occurring, then there should be a comma after "pony."
4. The sentence that begins: "He watched the fishes..." is troublesome, because we really aren't certain whose eyes are sparkling, etc. And the : seems entirely the wrong approach. Try making into a smoother sentence that clarifies this AND removes the : IE. "He watched the fishes with their sparkling eyes and flippity pink-skinned bodies as they slid through the leathery marsh grass, swaying in a dark hypnotic rhythm--the oldest and deepest of magic. Inward, outward, shells, sand, and souls."
5. The two last sentences should be (and they might be, but the forum parameters don't easily allow for it without separating with a blank line) a new paragraph.

I think I addressed more than just grammar, and other than the few punctuation oddities, I found nothing objectionable about the grammar. Besides, grammar rules are somewhat flexible when you wave you're creative license. You'll find yours in your HUB (Hatrack Utility Belt).

As far as the prose...umm...when do I get to read?