Very nice film. I especially liked the anemonefish tending its eggs, and that black manta is beautiful.
As you're looking to improve, I did notice one or two slight wobbles at points that I might have tried to stabilise or cut in post. Also I think you may have just slightly overdone it darkening the footage in a couple of places. The blues were occasionally very deep, almost too deep, and the blacks very black. But I'm being very picky here.
One other thing. Be very careful with CC licences. You generally need CC-BY for what we do. CC-BY-ND doesn't allow for syncing to video. I love the track you've used and wanted to use some of that artist's music myself, but very little of it is CC-BY, and I've been too cheap so far to stump up the $99 for a Jamendo pro licence.