I think that's exactly the description of a good background; it can't be confused with the foreground sprites. Especially in a game which gets as hectic as a shmup it's vital that the background doesn't clash with the sprites.

Beautiful job all round. 

very smooth shading, good job

Glorious stuff :)

The water reminds me of Banshee on the Amiga.

Beautiful background, does a good job staying out of the way of the foreground (at least from what we can see from that one sprite )

Don't kick me, but I think if you remove the flying object (which is cool, no doubt) in the center, the whole image would look better, IMHO. They are too different, even though it's a background, I know, but still...

Well-detailed, I love it!

Your stuff is God-tier pixeling. God tier.