Old Toothbrushes - some "soft", some "hard" bristles,
differing numbers of rows in each toothbrush
Masking Fluid and a palette knife
Cheap Joe's Spatter Screen
In both watercolor and acrylic painting, I have used spatter and splatter to create texture, whether on pears, bananas (those brown spots), or in landscape for bushes, trees, "things" on the ground, and so on...
There are a number of ways to create spatter, but in watercolor, it is all about timing. If your paper is too wet, the spatter will be absorbed into the background. If your paper is too dry, the spatter sits on top of the painting, and looks "pasted" on. The ideal time to apply this texture is to wait until the shine has just gone from the paper, then apply your paint.
Spatter is often difficult to control. No matter what method you use to create spatter, make sure that you cover the parts of your painting that you do not want spattered. You can do this with a towel, paper towel, tissue paper, paper bag - whatever you have handy.
When you use a brush, you will get larger drops of paint landing on your paper, which is great for larger paintings, and larger areas. For smaller areas, or for more control, a toothbrush works well. Holding the toothbrush at an angle to the paper means it will spray the paint more or less where you want it, and in a more random pattern than if you hold it on top of the paper over the spot you want texture. As well, you will need to control the amount of water in your instrument of choice so that the pigment is light or dark enough, and will easily fly off the instrument onto your paper, but not be so diluted as to not make that texture you are after.
You can make positive spatter (paint being flung onto your paper) or you can create negative spatter where you leave the white of the paper in a spatter technique. You can achieve the negative spatter by placing some masking fluid on your palette knife, then snapping your wrist above the paper to fling the mask onto the area you want to leave a white spatter. This works well when you are painting ocean surf,the white of a waterfall, or waves crashing against rock. Again - make sure your paper is covered where you don't want the mask to land.
I have discovered a wonderful tool which makes precision in spattering more possible. It is Cheap Joe's Spatter Screen. These come in round, square, fine and medium textured. You can find Cheap Joe's site here, then just type in "spatter screen" for your search. With this tool, you brush on the screen the paint you want to splatter, then you hover over the targeted area on you painting, and gently blow the paint through the screen onto your paper. It is actually quite a bit of fun!
Practice makes perfect, and I recommend practicing over a scrap piece of paper until you get the ratio of water/paint and that wrist movement just right with whichever instrument you are using. And remember - even though spattering is an effective technique, too much spatter in a painting ruins it. So, try to practice restraint with this technique.