Correct, and using very slow CW (Morse code). And even then subject to errors, so multiple retransmissions to hope get the messeag.
If it is a salt water only floating antenna, or sonar, in fresh water you can use CB radio up to 20 m deep.
for short range use 5kHz
Indeed - the lower the better (3 to 30 Hz)
➜ which is pretty far away from the 2,4GHz of WiFi or BT...
some reading ➜ https://www.diva-portal.org/smash/get/diva2:1148743/FULLTEXT01.pdf