floating

First off: I really like the composition and love the colors, and the shallow depth of field works well here. Having said that, there are a few "but"s.

What I notice immediately is that the picture isn't entirely sharp. Looking at the shutter speed (1/256), that's not that surprising. You need speeds of above 1/200 to freeze human beings, but I imagine this seal moving fast enough to require higher shutter speeds. With the low-lighting conditions in these types of places, I understand how you can't go up to much with your shutter speed, but that's where your ISO comes in.

I understand, quite perfectly, the desire to want to keep the ISO value to a minimum. Using a Canon 5D Mark II however, I'm pretty sure you could up the ISO to at least 400 though without any significant quality loss. This would leave you free to use higher shutter speeds, creating a sharper image.

Lastly, I'm seeing little specks. Whether this is dirt on the glass panels of the aquarium or something floating in the water, it is unnecessary. For me personally it gives the picture an unpolished feel, taking away from what lies behind the specks. Photoshop is your friend here, these things can be easily removed by cloning and patching.

Well, I hope this is the kind of critique you're looking for, just know that I enjoy this picture a lot and that I put in my favourites for a reason <img src="e.deviantart.net/emoticons/w/w…" width="15" height="15" alt="" title=" (Wink)"/>