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.