Above is a corner of my palette with five premixed values of a blue-gray tone. When I showed it last someone asked how it was mixed. I simply made a big pile of the darkest version using ultramarine, a little ivory black and a smidgen of titanium white. Each of the other piles was produced by diluting that "mother" color with gradually increasing amounts of white. I could of course do this with any "mother" color I wanted and sometimes I have several of these "strings" of color on my palette when I paint seascape, but seldom when painting anything else.
Below is an example painted on my palette of the use of a double loaded brush. I dipped one side (corner) of my flat brush into a dark pile and the other side into a light pile. Now I have two different values (or if I want, two different colors) on my brush. Over on the right I pulled a stroke to show you the kind of mark such a double loaded brush will make. I then painted the little wave study using a double loaded brush. I reloaded the brush after every few stokes.
I have worked at getting this effect to work for a long time and really only figured out how to do it reliably quite recently. It takes some practice and experimentation to control it. I found a reference to Frederick Waugh using this effect. The observer noted that Waugh twisted ( twirled) his brush between his fingers as he worked. That puts the dark on top sometimes and then the light note at others. It will also cause a striation of values within a plane of the water. If you look at the sketch above you can clearly see that. It is important to have the right white when doing this, Waugh used Permalba, but I painted this sketch using RGH (link in my sidebar) titanium white, the Lefranc is good too, and Winsor Newton is slippery, but stay out of the student gradee paints or anything too stiff or crumbly..The important thing for this is that the white is slippery and and somewhat soft, be sure you get the right ONE. Sometimes I add a little stand or linseed oil to get it to move better.
When I am doing this I am thinking about how the various planes of the water are facing. I also pull the shadow strokes downward and the lights up from below. There are all sorts of little niceties of handling, brush pressure and edge control that can be explored with a double loaded brush.