I built myself some shoals from scratch, and from scraps, and they came out fairly well I think! So I figured I would share my method. I will need this for the next scenario that I am preparing.
The basic ingredient for the shoals is expanded polyethylene. You just want to cut it down with a sharp tool in tiny little pieces which have a flat base, and then are randomly shaped above. Tip: keep some models at hand, to cut your shoals a size appropriate for your ships.
You then want to cut some basic shapes in plasticard, and paint them blue. This will be the rough area around the shoals. Does not need to be precise, on the contrary I find it the less precise the better, but you may want to give them round edges. You then paint them blue and use some white glue such as vinavil to glue your rocks on.
When the glue is dry, you roughly cover your shoals with plaster (stucco). Again, no need to be precise. A rough coverage, showing some of the polyethylene's texture in some areas, will look more natural.
You then paint the rocks with a nice dark grey, and apply the Vallejo water texture (Mediterranean in my case) on the water surface, sponging it.
When it is dry, you give your shoals a drybrush with a lighter grey and apply some light blue, and then white above it, around the shoals. Look at my basing tutorial for how to do that, very easy. Tip: in this case, less is more - remember the shoals are not moving in the sea, so keep those breaking waves small of they will look out of proportion. And that's it really, you have your killer shoals for free!