Plein air painting in Florida means that at some point you will be painting water in motion, and probably at the beach. Painting anything in motion is a challenge but water can be particularly tricky.
It's best to watch the waves for a short while and see what patterns they are making...do they come straight into the beach? Do they angle to the side? Do they peak right before they crash on the beach? Note the direction, and this will help you design your painting.
First, determine your horizon line, as it is crucial in capturing depth:
Next, notice that the waves closest to the horizon are thin lines that run parallel to the horizon. As they move closer to you, they become spaced farther apart and taller (darker). Waves closest to you reach maximum height and crash on the beach.
This reference has very small waves as its the Intercoastal and not the open Gulf.
Notice the pattern of waves is moving toward the side of the beach so they crash obliquely on the shore instead of dead on.
Also notice that each wave has a crest (Dark side) trough (light area) and the ones crashing on shore have a white froth and highlights. You paint waves just the same as you paint ripples in fabric --- with at least three values; dark, medium, and light.
notice the similarities to fabric? Most lake waves are gentle ripples like this. You can capture them easily keeping the idea in your head that they spread farther apart the closer they are to you and become more like thin lines closer to the horizon.
In pastel, I sometimes just side stroke the whole shape of the waves by the horizon, and make thinner lines for the waves in the middle. The waves closer to shore are reflecting some of the shore color and the shallow water color hence the brownish green, and have light highlights on the top.
Try to keep it simple. The more simply you paint the water, the more elegant it reads. Now in this composition the waves are not the main focal point or subject so I downplay them a bit. Here's another example. Notice the waves follow the same pattern as do the clouds in the sky.
Now when painting waves as the main subject, I prefer to play up the wave closest to the shore, and have it peak above the horizon line. Notice the waves behind it are nothing more than lines in the bachground.
Here the waves pulls you toward it, the dark crest draws your eye to the white highlights on the froth. You want ocean waves to be more dramatic and have a wider range of values.
Hope this helps in your wave painting expeditions!
Happy painting!