Is $20 your target cost or the MSRP?...... The sensors will cost in the region of $20 .......
You might be able to have a BLE or RFID based sensor at that price that would work fine at the surface as a proximity sensor (one of the typical application of BLE or RFID .... nothing new here ... and in fact we are using it in that way in our new No Diver Left Behind project).
Very unlikely that you can build a proximity sensor that works underwater (with a range of more than few millimeters) for $20. Anything rated better than IP67 requires $$$