And here again some "how I did" with the pictures:

In this specific case I compared pictures of old, galvanized metal. After that I put them away and tryed to catch that cloudy, blotchy white hue over the dark metallic ground color.
As basecolor I used Citadel game Color, Boltgun Metal (or whatever else is close at hand; any dark metal color would do it). Then I applied very thin cloudy coats of creamy white and traces of dark and light rust. The forementioned technique allowes you to control the colors carefully. That's all!

For those prominent ripples, caused by currents... water moving below the surface, and up to the surface, I did not have any idea to generate that effect. Then I discovered by chance such unusual fabric in the textile shop when I accompanied my wife on her Saturday shopping (obviously you should do this from time to time...!)
And here what I did: Soaking a piece oft fabric, that has been given the right shape, with Future (Pledge) applied by brush. Waiting for about half an hour, then carefully taken away. What remains is a slight trace of ... ripples.
Hope you understand what I wanted to describe

All the best,
Peter