Sound. There are commercially available acoustic modems for underwater use.

ROVs do so routinely, at greater depths.

Why are you assuming it's wireless? ROVs use data tethers with the spool kept on the base ship. Or you could have a black box recorder attached to the sub by a cable that blows its connection and floats to the surface if it loses contact for more than a few minutes.

I can think of 10 different ways to maintain some contact with the surface and store some data about dive conditions. Honestly perplexed at all the people who are still buying into Oceangate's demonstrably bad way of doing things and just saying it's impossible to do any better.

Not to be crass, but wouldn't any low bandwidth sound-based protocol be easy to rig up for underwater? Obviously there's a bit of adaptation required, but it shouldn't be _that_ hard to set up a 28.8k modem with a directional speaker and microphone rigged up to point vaguely toward each other (i.e. at the surface pointing down and on the sub pointing up). Hell, even automated Morse code sending literal text messages at 3 sps is better than nothing.