There are wire guided torpedoes and missiles that carry a spool with them and pay it out as they travel. Presumably this is a good thing because it can't be intercepted and jammed. Anyway, that missile carries 4200m of wire, which would be plenty for an ROV to explore most of the cave in the article before sending people, who could follow its wire.

https://en.wikipedia.org/wiki/BGM-71_TOW

You can't touch the sides in cave diving. It generates silt which destroys visibility.

Yup that's fine. If the ROV kicks up any silt, shut it down and go for lunch. Nobody's air is running out so you have extra time.

There's no current down in these caves. The logical solution then is to drop a trail of breadcrumb beacons to relay your signal along, either via light or some radio band that penetrates water halfway decently.

Are you certain? I've spent most of my dives in Florida springs, which can have flow rates above 100 cubic feet per second. You can basically get blown out of Devil's Eye at Ginnie Springs if you're not careful with your trim when ascending.

The downside of still systems is that any silt you stir up can kill the visibility for hours / days, which would cause problems for light-based beacons.

I wonder how much bandwidth a sonar link can provide over reasonable distance.

The a highly restricted, complex cave environment with echoes all over the place? Probably not much.

I suspect you could use the echoic environment to your advantage with a MIMO type system.

The ROV can drop an amplifying transceiver every so many meters?

How do divers communicate over long distances? If they don't, that's something worth solving in its own right. If they do, couldn't the ROV use the same thing?

Light signals. You can say 3 things over any distance, which are "OK" (as a question or a response), "attention please" and "OMG OUT OF GAS NEED HELP IMMEDIATELY".

Given clear visibility, which gets rarer and rarer with distance - and even more so in constrained spaces, which are unlikely to be pure bedrock. Also, line of sight.

Indeed. So you see operating a ROV like that is a non-starter.

I never said otherwise; are you sure you are not replying to somebody else?

They don't. GL;HF.