Once the sea and the sand were looking okay, the next step was to add some white foam, where the waves lap against the sand. I took the cloth I use for wiping my brushes (rather than a brush), put a little titanium white on my palette, dabbed the cloth into this, then touched it onto the canvas in short, sharp, tapping movement.
Where there was a lot of wet paint on the cloth, this produced a solid area of white; where there was just a little, it created a softer area. Keep dabbing with the cloth as the paint on it gets less and dries, to soften the effect.
Another option would've been to do this using a dry coarse brush with just a little paint on it. To do it that way you dab the brush into a little bit of thick paint, wipe off most of the paint either on the palette or on a cloth, then splay (spread) out the hairs before putting brush to canvas. Again, dab up and down with the brush onto the canvas, rather than spreading the paint across.