1. Create a new document, pick Gradient tool and do as the picture below:
I used green and blue color, like water
2. Use Pen tool to draw a blob like a balloon and fill it with green light color
3. Use Regular lasso tool to make a selection on the top of the ball and fill it with green color lighter than this ball
4. Use airbrush tool, choose the fade of tool is "fade" 35% and do as the picture below
5. Use brush with small size, white color and click around the bottom of the jelly fish
6. Use pen tool to draw some tails of the jelly fish like this
7. If you don't like the color of this background, you can change it to make it more nature, this is my final result
