Watercolors are often painted on the paper in transparent watery layers called washes. The most interesting effects can be achieved by dropping a dark wash on one tone into a still wet lighter wash. This works really well for clouds. The same technique was used on the surface of the water in the canal.
To get a cloud simply leave that area white. Use only clear water to start your wash and cove the area you wish to work with. To get a cloud to look real, simply leave that area with the clear water void of any color. then apply a light wash of color while the paper is still damp. Drop a blue tone with plenty of water in in and allow it to bleed into the wet paper. Then use your large round brush to gently blend it in. The thinner the wash the more bleed you get.
Drop a darker tone into the first one before the first one dries. The second tone will bleed right over the first one if it is to thin. Make your second wash application with a slightly thicker mixture of paint to water ratio.