I'm happy to stand corrected. However, zero visual visibility doesn't seem like a limitation for a remotely piloted robot. Can't they just as easily use sonar

I'm happy to stand corrected. However, zero visual visibility doesn't seem like a limitation for a remotely piloted robot. Can't they just as easily use sonar, radar, or some other form of non-optical "vision"? Fishermen use "fish finders" which are non-optical, so I'd guess the tech is pervasive and cheap.