|A small study of a pretty, sunlit wave at Praa Sands|
From a photoraphic reference
Watercolour (with masking fluid)
There is probably several posts worth of thinking to come up with related to that, though.
In the meantime, meet masking fluid:
You can, of course, just leave the area untouched - but that isn't always easy to do if your watercolour is wet and washy, or even if you just forget in the splashiness of the moment. Masked areas have hard edges, which may or may not be required (they can be softened with water), but which can be hard to achieve without masking.
Anyway, this is my first ever bottle of masking fluid. So far, it's proving to be really rather effective. Good for foam on waves and highlights on bottles...