> Would not a sonar help?

Portable sonar systems with in-helmet displays were put in use by the US Navy only last year: https://www.navalnews.com/naval-news/2021/03/us-navy-teams-w...

> A full spacesuit-like helmet with air to be able to talk via radio?

Would fuck up your buoyancy, but there are systems where you have only an air pockt around your mouth. But the range would be so low as to be nearly useless inside a cave.

> Laying down some flexible tunnels that could be inflated with air or maybe just pumped with clear water to gain visibility?

Even assuming that you had those already available, laying them would be a very slow process, and they would be a hindrance rather than help in very narrow passages.