I like this. It's crying out, imho, for a horizontal-scrolling parallax animation. And moving bubbles. And make the seaweed wave about. And some sprites in the foreground. I guess what I'm saying is that it looks like a cool game background, and I'd enjoy seeing a full mockup.
Oh, and I second everything Chris said.
. it would be even cooler if it had the underwater current effect animation (like in DK country, and, well, most any underwater level 
).
This looks almost pillowshaded. I like those colors though.