[naikrovek]

water absorbs 2.4GHz readily. this is why microwaves use that frequency to heat your food. microwaves are why that frequency band is unlicensed, too.

so that's Bluetooth and wi-fi ruled out.

GPS is a lower frequency, ~1.5GHz I think, and GPS is already an extremely low power signal.

I don't know about 4g or 5g though.

[kelnos]

4G and 5G are going to depend on the mobile carrier. In the US, at least, that's usually going to be somewhere between 850MHz and 2100MHz. I recall reading something more recent about 700MHz being opened up in some areas.

Looks like Germany is 700-2600MHz for 4G, with 5G up at 3500MHz[0].

For underwater stuff it looks like you need something much lower in the kHz range[1], at least for distances of up to a couple hundred feet. Obviously this particular situation involves a much shorter distance. The page on MF radio[2] does mention water, and talks about frequencies up to 3MHz, but that's still way lower than any LTE bands used.

[0] https://www.gsmarena.com/network-bands.php3?sCountry=GERMANY

[1] https://en.wikipedia.org/wiki/Communication_with_submarines

[2] https://en.wikipedia.org/wiki/Medium_frequency

[ac29]

Verizon has used 700MHz for a long time, its their primary band. T-mobile uses 700MHz and 600MHz in many areas (generally as range extension, not a primary band).