An ultrasonic touch sensor includes a touch structure including a touch surface configured to receive a touch, a signal generator configured to generate an excitation, a capacitive ultrasonic transmitter configured to transmit an ultrasonic transmit wave toward the touch structure based on the excitation signal while the touch surface is submerged under the water, a capacitive ultrasonic receiver configured to receive an ultrasonic reflected wave produced by a reflection of the ultrasonic transmit wave at the touch structure while the touch surface is submerged under the water and generate a measurement signal representative of the ultrasonic reflected wave, and a measurement circuit configured to perform a comparison based on the measurement signal and a threshold, and determine whether a no-touch event or a touch event has occurred at the touch surface while the touch surface is submerged under the water based on whether the measurement signal satisfies the threshold.