In watercolor, the paper is the whitest thing (unless you paint acrylic or Chinese white paint over your work). You have to leave the white places showing until you decide where you want your main “accent”, focus, or center of interest. Your lights should wiggle in and out of the picture plane but not be a deterrent and take away from other more interesting features.
Especially when painting boats, of course you want to have them white, but choose which one you want to be the whitest and drop washes of watered down colors into the others. Back painting darker shades will also make things lighter in front (negative space). Paint a darker shade between the grass blades or flowers to bring them forward and brighter and lighter.
Look at these boats, they are white but the contrast and the shadows bring out different places. You can’t really adjust the whites in a picture, but taking them on sunny days provides the opportunity to capture the whites with shadows. Look at the deep shadows below. They are essential in paintings.
I have noticed that some new painters do not ground objects with a shadow. Use a light source and see where the shadows lay. It is great to show a color or two reflected in a shiny surface (like metal or glass) even in a window, a touch of a color repeated from somewhere else adds interest.
Shadows can create angles to bring the eye into the picture. So do clouds coming in on an angle instead of white little puff balls. (Go out and look at clouds, I take pictures all the time to remind myself that all clouds are not the same.) The problem when painting from a photograph is that in the 125th of a second, a real shape gets snapped. If you watch clouds, they are constantly changing so soften the edges to make them fluffy, do the same with shadows. Wet the edges as you paint them in to create this effect.
Have fun painting or shooting for the whites and don’t forget your shadows.