I'm looking to measure distances from a submersed robotics project in the range of 1 to 20 feet (around there at least). However, the only options I seem to come across are rather heavy sonar modules that cost hundreds of dollars. Are there lighter, cheaper alternatives? I don't need great resolution or accuracy, just a decent measurement. Perhaps it's possible to waterproof a normal distance sensor (IR or ultrasonic)?
I intend to interface the sensor with a microcontroller so it would be nice to have something that outputs 0-5 V, but that's not a requirement.