I haven't done a lot of research, but https://en.wikipedia.org/wiki/Hadal_zone seems to suggest it really has only been a handful of people; Jaques Picard and John Walsh, James Cameron, and a few Chinese and Japanese explorers; even unmanned craft struggle to go down that far it seems.

Which... is kinda weird to me, like with space missions, I'd expect them to launch probes or permanent unmanned observation things down there. Surely they can transfer data through water efficiently enough?

I can imagine they don't want to use nuclear power (like a RTG) on those though, which limits lifetime. A cable going to the surface will be difficult as well, because of its own weight and currents.