Thanks Steve. It turned out better than I expected although I deliberately 'dimpled' the water surface in a way that produced a realistic specular effect once it was varnished.
For anyone who is interested I found what worked was airbrushing the water and wake effect on top of the royal blue base colour, working dark to light. So the final touch is just white airbrushed to create the churning water, followed by clear Tamiya blue over that layer, and several coats of cheap craft store varnish.