I've painted animals on rocks before, but never tried fish. Out of the gate, it didn't go that great. I needed a fish anatomy lesson! The fish refused to conform to the norm and ended up looking a little wild.
Hey, that's Art! :)
First I got a thin board from Lowes lumber department, sanded and then painted it with blue and green paint. Using a water color technique to give it a washed, watery look, I let it dry while I worked on the fish.
I gave up on fish-realism and went with the wild, whimsical, dressed up kind. They were a work in progress as I layered on paint, stripes, and polka dots. Hippy fish!
When the board was dry I took some sea shells my daughter collected at the beach and used Turbo Tacky glue to attach them across the bottom. To fill in between the shells and coral, I used sea glass purchased at Michaels. I let that dry over night. 
Then i went back to the fish. Then the sea shells all over my work bench sparked an idea. I decided to use them as the fins of the fish, giving the whole mixed media piece a little more depth. I put a light tint of paint over the shells, let them dry and then glued them on with Turbo Tacky glue. When the shells were attached, I used the glue to put on the fish on and let dry over night.
To hang it I screwed in eye-hooks on the top of the board strung some burlap string through the eye hooks and attached sea shells.
Go fish! What are you working on?