Under the Sea

Hey Rose, Matt here.
Gosh I've been running around preparing to fly over to China so I'm extremely sry for the late reply!

So the first thing that hit me was the lack of a light source. Or if there is one, it's not obvious enough. The picture will look kinda flat if there's a lack of values.
I went back to check out your sketches and they seem to be generally okay. So I'm assuming that it's because you're not used to colors

So give it a good strong light source, taking note how light looks like in water. I recommend googling for underwater pictures to see how light looks like in water.
Make sure everything in the picture corresponds to the light source. Take note of the reflective lighting as well.

Also keep in mind that the environment color will affect the colors of the fishes etc. E.g. how metal will reflect close by colors.
So even the seaweeds will take on a blueish tinge thanks to the water. It won't be purely green even though your brain tells you seaweed = green.
Once again, if u can find reference pictures, they will help you alot. It doesn't have to be photorealistic of course, since I know you're going for semi-realism.

Next I'll touch on distance. The objects further away from us should appear lighter than those in the foreground. Aka Objects nearer to us are darker.
This gives a perception of depth to your art piece. So you can make the fishes and/or the seaweed in the back lighter/slightly blurred out while the ones in front are darker and have more detail.

The signs have a rather odd perspective. I'm assuming the horizon line is at the top third of your page.

Right so generally you've got the basic colors down. all you have to do is spruce it up abit and it should shine! Of course feel free to ignore all of the above if you're going for a more cartoony look than what I'm describing. Hope that my suggestions help out and I'll talk to you soon!