It's a nice piece for sure, but there are a few flaws. They're not huge, but overall I think they do lessen the impact of the total piece.
First off I'd like to say you have the bones for a great manipulation here, I commend your vision (:
Now one thing I must mention is the proportions, they feel a little off to me, like the horse is a model horse or something O.O I think it's because of the way the background is blurred.
The colours are okay, though I think the horse could use a few more blue-tones near the belly, because of the water there you know? (Great work on the reflection by the way!)
Then I also feel there could be a few more splashes around the legs, as the current does look fairly strong :3 Some higher reaching splashes would be perfect
Nice work on the mane and lighting is nice too, I think the tail could use a few more loose strands though.
The marking on the neck looks a little flat, maybe try using dodge/burn or another technique to tone it up a little?
Also cutting looks a little sharp around the chest and neck, maybe smooth that out a little?
Overall I think it's a nice job, but a few minor edits would make it look just that much more stunning