Boater caps were headwear of straw worn famously by boaters.
Yasmin Khan wore boater caps done in navy blue, (TV: Nikola Tesla's Night of Terror) or light brown. (TV: Survivors of the Flux|The Vanquishers)
Leela wore a light brown boater cap when she and the Fourth Doctor reached Fang Rock. (TV: Horror of Fang Rock)
Missy wore a black boater cap. (TV: Dark Water/Death in Heaven)