As simple as this is, I think it's really great. I like the relaxing feel. I agree with Danori though, the cloud could use just a little bit of work.
I noticed you used alotta colors for the ocean and sky, but only two for the cloud. If you require no color restrictions, why not give the cloud more colors? It almost looks like it doesn't belong due to the limited amount of colors on it :p
Uau, I really like the water in here! You're improving from piece to piece, good!
I think the cloud needs a little bit more shading. It creates a contrast to the other parts of the image which are more detailed and dithered. But I really liked the waves and the colors.