• Never paint over a white background! Before you start, paint the "sheet" with some neutral tone, like a desaturated beige or gray. If you are going to make an ocean or a night pic, you can use some desaturated purple/blue as a base.
Example

• Use hard brushes. Airbrush gives a "too perfect" look for things, while hard brushes make things more organic.

• Use low opacity, like 10%.

• Send your "history" to hell! Paint fast and loose, let details and other stuff to render later. Stroke to right, then left, then diagonal, and keep going, you will see many colors and gradients form naturally. Go crazy, this is CG - you can paint over and over without ruining it!

• Change your color, use purple, gray and other tones (remember that ocean reflects the sky) and change your brush size too.

• The rendering is up to you. You can remove the brush marks, make foam details... you can use reference here, if you want a more realistic painting.

• Also, check out this site: www.goodbrush.com, and go to the "skecth" session. You will see, it's all brushes! Good luck!