Non-Abrasive Liquid Level Sensor

Hello, I have a 316 stainless steel cylinder that is 3 inches in diameter, 17 3/4 inches in height, and 1/16 inch wall. A linear actuator oscillates a float inside of the cylinder to displace volume and raise the water level up as it submerges. As water slowly escapes the cylinder, the level of the liquid will drop. I need a sensor (non-abrasive) to penetrate the steel wall, travel through the liquid, hit the float, and return. If no water is sensed in the cycle, the sensor will act as a switch to turn on the linear actuator so that it brings the float down (linear actuator on the outside rides a magnet up and down - the float inside is held down by magnets at the same level as the outside magnets). Does anyone have any suggestions? I am searching around for ultrasonic transducers for the job. Thank you, Shant