For ultrasonic communication or echo ranging to work under water, you need transducers designed to couple the acoustic signal to water, rather than air.
They tend to be quite expensive and require several hundred volts AC at typically 150 kHz to drive.
The transducers in the cheap "Lucky Fish Finder" work well and give up to 100 m range, but the circuit used to drive them is not easy to modify or build. Lucky Fish Finder New | eBay
There have been some attempts to make an inexpensive underwater acoustic modem, but none seem to have made it to the hobby market.