This tutorial was taken from the New Entity
2. In a 2D view, drag out a box the size you want your water to be.
Adjust the size in the other 2D views.
3. (This step is not necessary for most other entities) Click the
“Browse” button in the texture settings (under the tool icons). When
the viewer pops up, change the filter to “water” (without quotes).
Select one of the materials.
4. With your mouse cursor over one of the 2D views, press Enter. The
brush should be selected if not, select it with the selection tool.
5. Click the [[mapping:cawe:editingtools:newentit|New Entity] tool
6. On the far right side, select “func_water” from the drop down box.
Click “Turn into (solid) entity type”.
Now compile your map, and you should see water!