It can be hard enough for a human to not get stuck somewhere in a cave. There's little chance you'll figure out how to get a robot to do the trip before the whole situation is resolved anyways.
In such situations it's best to go with the simplest solution you know will work and people are trained for, instead of trying to get fancy and possibly endanger the whole undertaking (imagine a robot getting stuck somewhere and blocking the way for everyone...).
I don't know anything about that cave, but it's quite common for cave divers to remove their equipment (and put it in front of them) because a passage is so small that the diver and a gas tank won't fit at the same time. What's the chance of a robot doing the same?
The situational awareness of an ROV operator (who can only get visual feedback (in very low visibility conditions) is far lower than a diver (Who can feel their way along). Way too easy for an ROV (which has to tow along a tether) to get itself tangled somewhere.
I am by no means a subject matter expert, but I'd expect sonar would be next to useless in turbulent water, close to a bunch of highly reflective surfaces. Sonar also doesn't solve the problem of a kilometer long tether getting tangled at some point.
In such situations it's best to go with the simplest solution you know will work and people are trained for, instead of trying to get fancy and possibly endanger the whole undertaking (imagine a robot getting stuck somewhere and blocking the way for everyone...).