This is a seascape exercise I've had kicking around the studio for a while, from a previous workshop. Upon a second look, I 've decided that it would probably benefit from the addition of a boat as a focal point.
First I design a simple boat, in sketch form.
Then I re-draw a rough version of the boat onto a piece of lightweight cardboard (cereal packet card is ideal).

It's a good idea to do this in place, on top of the painting, enabling you to get the size right relative to the rest of the composition.

Once the sketch is complete, I carefully cut around the boat with a sharp blade (a scalpel or craft-knife). Be sure to do this on a suitable surface (here, I've placed it upon a sheet of MDF).
I then press out the boat shape.....
.... leaving a boat-shaped hole in the card.
Place the card in position on the watercolour.
Once in position, scrub the surface of the watercolour, through the cardboard 'mask' with the Wonderbrush.

Note: The Wonderbrush should be damp with clean water.

How easily the paint will scrub away will depend upon several factors. Some pigments are more permanent than others and will take a great deal of scrubbing. The age of the painting will also have an effect. This painting had been lying around for quite a while and was, therefore, quite stubborn! Freshly-painted areas will always lift out easier.

Once the shape is fully scrubbed-out, dab it dry with a piece of tissue.
The resulting highlight can now clearly be seen.

It's important to note that scrubbing out with the Wonderbrush will not always remove all the pigment - care should also be taken not to scrub so furiously as to seriously damage the surface of the paper or the sizing (different papers will have different results).

Once dry, I begin to paint in the hull of the boat.
I'm using a Cadmium Red for the light side of the hull, and a mix of Cadmium Red and French Ultramarine for the shaded side.
The finished hull.
I use a dark, rich mix of French Ultramarine and Burnt Umber to paint the boat's windows.
The finished boat.
I now begin to lighten selected areas around the base of the boat, once again scrubbing with the Wonderbrush (damp with clean water).
Once dry, I apply a light Burnt Umber to the sand, leaving the lifted-out areas as pools of water.
Next, paint in the reflection of the hull, first with the Cadmium Red...
.... then the darker, Cadmium Red / French Ultramarine mix. Remember reflections lean the opposite way to the object (I know - it's obvious, but you'd be surprised how many students get it wrong!)
I'm now applying a shadow of the boat to the sand, mixed from French Ultramarine and Burnt Umber.
As the reflection of the boat in the puddle would be as if you were looking at it from the surface of the water, what you would see here is the blue of the sky. Here, I've used Prussian Blue.
Finally, I'm going to give the boat a mast.

Place two straight-edged pieces of card close to each other, as shown, and scrub through the gap with the Wonderbrush (remember to dampen the brush first, with clean water).

Taping the card down with a couple of pieces of masking tape helps keep the card in place. Failing that, an extra pair of hands, would be useful!.
Finally, a few boaty-rope-type things...
.... and here is the finished painting!