There isn't a limit on the water, correct? If my party had one of these and they're doing Tomb of Annihilation, this would be priceless.

If you have a Brazier of Commanding Fire Elementals, then the Decanter is MUST HAVE for controlling her when you lose concentration...!

If you want to be tricky as a DM - give this to a player but don't inform them that there is no way to turn it off. It doesn't say anywhere in the description that it can be shutoff.  You'd have to drag a running fire hose everywhere with you after you turn it on!

It produces a set amount of water depending on what word you say, it doesn't need to be shut off at all

"The water stops pouring out at the start of your next turn." It shuts itself off. Plus if removing the stopper is a requirement for the magic to happen then putting it back on would logically make it stop.

As a Water Genasi, this is the most powerful thing I can have for myself or for other water-based characters. YES.

Soverign glue + Decanter of endless water + ring of regen + Bullying your dm = infinite regen potion

Quote from Jock_Tamson >>

These volumes seem off by a order of magnitude. 
30 gallons is a bathtub. 
A 1 foot wide stream of water that fills a bathtub in 6 seconds is not exactly a geyser. 

That's just the activation word. 30 gallons in 6 seconds is still compared to a smaller fire hose which spews 300 per min so it makes sense that it can knock someone off their feet

Fun probably useless fact:

If the 30 gallons of water stretches out over 30 feet in a 1 foot wide line, the "geyser" has to be just a bit more than an inch and a half tall. It seems more like a water blade than a geyser. 

NOW WE'RE TALKING!! Now it becomes a brutal as hell weapon

How would you account for damage to a fire elemental?

Quote from Ashwillow >>

How would you account for damage to a fire elemental?

 Someone mentioned it earlier. 1d4 divided in half for bludgeoning damage, plus 1 damage for each gallon of water that hits it.

Hrmmm,   Seems this can be used Multiple times a day. There is no rest period

Would you all agree ?

I'm using one as an Artificer in a campaign set in Lamordia. So my hand crossbow is magazine fed and gas powered (explaining crossbow expert/ repeating shot) and when traveling away from my shop, my air compressor is in a portable hole with a water wheel and a Decanter of Endless Water running it. It has a catch basin under it and a canvas hose I throw outside of the portable hole when in use to carry away the extra water. Need to refill the air cartridge on my crossbow? Just hook it to the compressor and say "geyser" every 6 seconds for a minute or 2 and there you go! Nothing extra mechanically , just some added flavor.

So I did some math and... 30 gal of water is 4.01 cubic ft or 6930 cubic inches (or 249 lbs)

I am wondering if I can get my Imp familiar to fire this. RAW i can't see any reason why not.

Quote from Jock_Tamson >>

A 1 foot wide stream of water that fills a bathtub in 6 seconds is not exactly a geyser. 

It's not a huge geyser, but it's 30 gallons every six seconds which you can sustain for as long as you can keep shouting "GEYSER!" so it's a pretty hefty volume of water over time. If you maintain it for a minute that's 300 gallons.

my first thought was to use it in pair with shape water to freeze the water on the ground

oh no

A fellow player in a campaign we're currently in has this item. He was not getting anywhere on an interrogation. So, I asked him to aim his Decanter at the prisoner's knee, and gave the prisoner a final chance to spill the beans. The prisoner (DM) rolled his eyes, said he wouldn't give up anything.

I nod to my friend, who unstoppers the raging water, upon which I cast Shape Water, causing the first 5 cubic feet to form a giant ice cube, which rocketed into the prisoner's knee.

The DM's jaw dropped, but he recovered, smiled and the prisoner sang like a canary, as well as taking a bit of creative damage.