I am drawing a silly picture for a friend. Its 2 people camping, and I added a river/water fall that splashes on a bunch of rocks. Here is the image I used as a reference photo, (It was NOT made in ArtRage though) I circled the exact technique that I am trying to accomplish

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(The white foam type part)

So I tried to put some white blobs around the rocks & then used the knife tool, & tried different presets like the frost/tiny frost but the white disappears & its not giving the look I want. Anyone know or can suggest any other ways to do this?

Btw I am using my ipad for this, that is why I posted in this section but I am sure any techniques will work the same on both computer/ipad.