This may be a somewhat silly question, but why couldn't they just have used flashlights? I'm sure there's an obvious reason, but I don't know what it is.
They do, but in zero viz there's so much crap suspended in the water that you literally can't see anything more than an inch in front of your face. Its the worst driving-into-a-snowstorm problem you can imagine.
I assumed it's not just darkness but mud.
I assume it wasn't just darkness they were dealing with, but also that the water was so muddy it became opaque, in which case lights wouldn't have helped them.