Thoroughly combine butter, brown sugar, and white sugar until smooth.
Add egg, vanilla, and water and mix until thoroughly combined.
Sift together flour, baking soda, and salt. Add to the wet ingredients. Mix until combined.
Add in the oats. Mix until just combined. The dough should be slightly sticky but not too wet.
Bake on a lined or greased cookie sheet for about 10 minutes or until the edges are set and the center is still gooey.
Notes
You may need to add more flour to the dough to get the right consistency. This is common in baking recipes and will depend on your altitude and the weather.