Wouldn't the cloudiness itself be a signal?

Presumably it's essentially clear of suspended particles, most of the time.

The issue is that the cloudiness of water, even open water, varies greatly. So there would be either lots and lots of false positives, or the system would be too insensitive.

Think driving in fog

A signal of what?

Something solid ahead

To clarify: I mean that, generally speaking, the presence of 'cloudy' water would presumably signify that the seafloor was close. In the open ocean, maybe a sea mount is surrounded by essentially a cloud of particles.

Edit: I think your reply made the thread reach maximum depth, so I'll end it here.. I should do more reading on the topic.

The problem I'm highlighting is it also gets cloudy randomly in open ocean, for lots of other reasons.

Have you ever dived? Max visibility is like 30-50m unless you’re in a filtered environment