The colours in the sea will vary depending on the depth of the water, reflections from the sky, shadows from moving ripples and waves in the sea, greater translucency in shallower water, and the white foam (many bubbles of air) of surf and wash foam and suds.
It may sound odd but sometimes we need to remind ourselves to observe what we are really seeing, rather than let preconceptions misguide us.
One way of achieving this is to turn your photo image and canvas sideways. The seascape image might then present to us as bands of vertical colour. We can then paint these bands first and return to paint the detail over them.
Tip: To paint a straight line, paint downward while moving your arm in a downward motion in order to move the brush rather than flexing your wrist downward. If you push hard on the brush your hand will be rest on it and thus be more stable than if you used a softer pressure.
A practical approach can be to paint the general background information first and add the foreground detail later. Hence the white surf and foam will be painted last. Also keep in mind that features in nature are often random, there is little symmetry and we find a mix of colours.
In order to work quickly and in multiple layers in one sitting it is critical to make your oil paint weak. To achieve this you will need to:
There are different ways to approach the white areas in the painting, e.g. surf and suds:
The following is simply a guide to the colours you might choose to paint a seascape.
Mix phthalo blue with zinc white. Add a tiny amount of alizarin red and lemon yellow, even a little ivory black.
Use some of the sky colour mix and add more blue, a little and red, black and zinc white to adjust.
At times the colour is lighter on the edge of the horizon.
Blend a little into the sky to soften the edge between the horizon and the sky.
Bands of colour:
Squint to see the average colours in the bands. Adjust colours and tones, eg. by variously adding yellow to make turquoise, blue for other bands, white and a little red if needed.
If a colour is too yellow add more red.
Blend edges of colour bands slightly.
Mix raw umber and zinc white, plus a little blue and red, plus medium.
To paint the sand push hard with a large brush.
Go back to your bands of colour and start adding some detail, e.g.:
Using a fine rigger brush and some of your Raw Umber mix with less white, paint shadows under the edge of the breaking foam and under the suds.
Detail: When your painting has dried you can go back and refine it, adding more detail and correcting areas you want to change.