One of my players, (a water genasi) has a Helm of Underwater Action- which for obvious reasons seems a little redundant.  However, as they do not have darkvision, they have been using the helmet (out of water) for the darkvision benefit.  Is this correct, or is the darkvision component only available when used in water?

All the effects are active all the time, though the others aren't particularly useful outside the water.  Though I suppose you could breathe out of a waterskin in a poisonous environment.

As Crawford says, it does what it says and nothing more or less (paraphrasing. It also may be Mearles).

"While wearing this brass helmet, you can breathe underwater, you gain darkvision with a range of 60 feet, and you gain a swimming speed of 30 feet."

If it only worked underwater it would say "while wearing this brass helmet under water, you can....."

But honestly, to prevent redundancies just give him/her goggles of night and let them give the helmet to another non-darkvisioned, non-swimming player.

My character is playing GofSM and we got this helm. Is there a way to add this to my inventory without purchasing it? Thanks. 

You can homebrew a copy of it for your own use, just don't publish it.