Does "breathe normally in any environment" include water?

Yes

But does it include environments like a cloaker being attached to you (or does the specifics of it rendering you unable to breathe counter the arguably more general rule of the necklace)?

Water is an environmental factor. A cloaker is neither environmental nor toxic gas.

If you put this on a goldfish could it live in space?

Quote from BossLogga >>

If you put this on a goldfish could it live in space?

 Now you’re thinking.

Would the space goldfish die of asphyxiation from "I can't breathe"? Nope.

Would it freeze because space is that cold first? Absolutely. But if you give it protection from the cold and the cosmic radiation then this item is an essential part of keeping your space goldfish alive. 

You obviously dont know enough about dnd space OT is actually a quite temperate all be IT airless enviornment where you have your own personal gravitational field ANd air bubble though you would need to be at least a giant for the gravity to be anythibg special 

Need this for my Kobold Ironman...

You could breathe in lava!!

I guess you could hypothetically, though this item wouldnt stop the lava from killing you

Yeah, I know. I wasn't actually serious :)

Okay, so DM gave us this item and no one in our party wants it.  It has a very niche use, and I’d rather save my attunememt slots for something a little more practical.  So how much would you be able to sell it for?  

This item is great for a nautical campaign, Though seems kinda niche otherwise.

Best use: give it to a Locathah PC. Get rid of that pesky "submerge in water every 4 hours of suffocate" bit

Yep. This is the niche. Very cool. 

Would this be able to be used in a space like a bag of holding where you can run out of air?

Quote from RagingPestilence >>

Would this be able to be used in a space like a bag of holding where you can run out of air?

 Personal ruling, but I'd say yes.  The environment in the bag of holding's pocket dimenion (or whatever it is) is harmless other than the fact that you'd run out of air, and this thing lets you "breathe normally in any environment," so you'd be fine in there until you started hurting for water or food (if you don't have those things with you.)

Basically, you can breathe normally anywhere, so long as your windpipe isn't being actively obstructed somehow (ie: something choking you, or a pillow over your face, etc.)

As it enables breathing in a vacuum, it renders the cloaker's smothering attack harmless 

Does it include lava?

like breathing under lava?