Gabe Newell, yes the owner of Valve/Steam, has an ocean research organization that owns a submarine designed to dive down to 10km, to some of the deepest parts of the ocean.

To be fair, even if no vehicle that could rescue it from the bottom is available, that doesn't mean that there is no use in a rescue mission if they don't know where the sub is. For instance, it could have surfaced but had it's communications lost, in which case they'd still need rescue, because based on a CBS video of the sub in question shared elsewhere on the thread, the thing doesn't actually have a hatch and just has the front taken off and then bolted back on afterwards to let people in and out, and obviously is going to be airtight given it's a sub and all.