Preheat the oven to 350 degrees F (175 degrees C). Grease cookie sheets.

Stir together flour, baking soda, and salt in a medium bowl; set aside.

Cream together brown sugar and butter in a large bowl until light and fluffy. Stir in egg, water, and vanilla until combined. Mix in flour mixture, then stir in oats. Drop by teaspoonfuls 2 inches apart onto the prepared cookie sheets.

Bake in the preheated oven until lightly browned and edges are set, 12 to 15 minutes. Transfer cookies to a wire rack to cool completely.