Underwater acoustic modems have been available for some time.
This omnidirectional model is capable of 64 bits per second, 200 m range and costs $2000. You will need two, of course.
I have seen some projects using parking sensors from cars used underwater. As they are sealed and sometimes operate with water or mud on them (obviously reduce effectiveness) they have provided reasonable performance underwater.
windoze_killa:
I have seen some projects using parking sensors from cars.....reasonable performance underwater.
Interesting. Got any links?
(Thread has slightly derailed, but the OP seems to be AWOL and the problem and goals are still ill-defined, so...oh well...maybe this will help.)
I will see if I can find them again. It was a while ago.
Thank you guys for your contributions,
I'm based in Germany, sorry for mixing up the frequencies, for me everything below UHF is considered as low-frequency as I play around with LoRa a lot.
For under water transmission, sound or ultrasound might be a good idea but I'l need some out of the box ideas for that since there are no off-the-shel modules for that I guess.
Anyways, your contributions are helpful.
you will not be able to use air based things for under water.
you can mount a relay on the side of the pressure hull and activate it like Morse Code.
you would need to be able to listen to the tapping in the other device.
look at the old telegraph key. instead of an anvil. mount it so it hits the pressure hull.
as a note: lots of research and probably links to sensors by searching for dolphins.
"The frequency of the sounds produced by a bottlenose dolphin ranges from 0.2 to 150 kHz. The lower frequency vocalizations (about 0.2 to 50 kHz) are likely used in social communication. Social signals have their most energy at frequencies less than 40 kHz. Higher frequency clicks (40 to 150 kHz) are primarily used for echolocation."