How does darkvision underwater work?

I'll assume it's about the same in clear water but reduced in murky water. Is there anything else about this ability I should know? Thank you.

Also, when throwing an unconventional object as a weapon (ie. plate, rock), how do I calculate range and damage? Thanks again.

The way darkvision works underwater is unspecified, which probably means it works the way it does above water -- it negates obscurement due to darkness but not other causes.

Unconventional objects use the rules for improvised weapons.

The biggest thing to remember IMO, which I frequently forget, is that in darkness, Darkvision gives vision as if in dim light. 

If you dig deep enough you’ll eventually find this:

Underwater Visibility (https://www.dndbeyond.com/sources/dmg/adventure-environments#UnderwaterVisibility)

Visibility underwater depends on water clarity and the available light. Use the Underwater Encounter Distance table to determine the distance at which characters underwater become aware of a possible encounter.

Darkvision does nothing against murk. The only thing I know of that counters murky water or murky air (fog/smoke) is blindsight. 

A dolphin would totally ignore the murky water and ''see" as if it were clear, at least within 60 ft.

Thanks for replies. Seems like common sense.

I know this is an old thread, but in case anyone else finds this while looking up the question, I'll highlight this; in clear water with bright light a character sees 60 ft in water. So, if a character is looking in clear water, dim light, with 90 ft of dark vision, they would see 60 ft because the dark vision negates the dim light effect only. In looking this up I have seen many people who supercede these water visibility guidelines with the rules for dark vision, allowing characters to see more than 60 ft in water.