Make a new image with your texture (32x32 normally). Hold the mouse down on the "Styles" colour swatch until the popup appears, choose the third option. Click to choose the texture, yours will be at the top.
To fill the water, flood fill the pink to black, then magic wand the black water areas. Expand the selection by 5 pixels. Then use the colour-replace brush to replace black with your water texture, double-click to replace all rather than painting over.
It's easy when you know how but hard to describe...
Cob Challenge
