Start by creating an ellipse.

Press Ctrl+T. Right click the layer and select warp.

Adjust nodes as shown.

Apply 2 pixels stroke to the ellipse.

Using pen tool create the tail of the fish.

Add fins to both sides of the fish using pen tool.

Add an eye brow using pen tool.

Select ellipse layer. From layer menu select layer style>>create layers.

Select the ellipse and stroke layer. Press Ctrl++E to merge.

Select eraser tool and start erasing the overlapping areas.

Create a nice pattern on the fish body.

Using elliptical shape tool create eye for the fish. Using pen tool draw ellipse and fill with a nice bright color.

Apply a nice sea color gradient in the background.

Using line tool create some horizontal lines.

From filter menu select distort>>wave.

Apply settings as shown.

Reduce the opacity to 30%.