So, basically a controllable pressure fire hose?

On hell of a waterskin

I would love this IRL

The best super soaker of all time

me and my dad steal this from my brother, place it underneath him (or above him), and trick him into saying geyser. As a result his character, Woody, essentially always has a close watch on his decanter of endless water.

...my first idea was to attach a couple of them to an amphibious character's armor to use as propulsion in underwater battle...

This would be an excellent weapon against a Fire Elemental.  For the cost of 1 Action and 1 Bonus Action, you can aim it at the Fire Elemental and speak the command word "Geyser" and do 1d4/2 (/2 because of bludgeoning resistance) bludgeoning damage on a failed DC 13 Strength saving throw (the Fire Elemental is Immune to the Prone Condition) plus do 30 points of cold damage due to the Fire Elemental's Water Susceptibility.  That's just a bit more damage than hitting a Fire Elemental with Create Water cast at 3rd level would do without taking up a 3rd level spell slot.  With a max of 156 HP, that's not bad.

Water Susceptibility. For every 5 feet the elemental moves in water, or for every gallon of water splashed on it, it takes 1 cold damage.

Quote from Eldermage1 >>

So, basically a controllable pressure fire hose?

This is exactly what I thought. I really want to see if a DM will let me take this item instead of an Alchemy Jug as an Artificer Infusion, so I can play a Brelish siege engineer who found a better way after the war... B.G. Redmane, Sharn's First Firefighter. Or possibly just "Big Red", if they're Warforged... I wonder if I could wrangle the infused item as an Integrated Tool.

This in a party where someone can use Shape Water... so many possibilities!

Really interesting item for someone who doesn't have much to do with their Bonus action! My Warlock is going to see how often this can be to our advantage!

EDIT: Ok my bad. I got too excited after this dropped in our game and didn't pick up on the fact that weaponizing the Geyser requires both an action AND a bonus action, making it not very compelling at all in most situations. Woops. Anyway the stuff below still might be helpful if you plan to use the geyser attack on a regular basis. 

Doesn't show up in DNDBeyond as a Bonus Action, hope they add it soon when you equip the item! 

BUT I managed to add it using a Custom action! Screenshots show the setup I used and how it looks after. 

If a paladin or cleric uses Ceremony to Bless the water inside the decanter, do they have a limitless source of Holy Water? Additionally, if so... would the 'Geyser' command inflict Radiant Damage to Undead and Fiends? Perhaps as a 2nd die?

If you were on a boat... and it was on fire... should you use fountain or geyser to put the flames out? This boat is huge, by the way. The 30 ft. From the geyser option isn’t long enough to cover the whole boat.

If someone is bound, can you force the opening in their mouth and give the geyser command to drown them? 

Tapes two on the back of a Tortle

Blastoise

Yes it would. An easy way is to use Sovereign glue to afix a blessed ring to the opening and it essentially is an undead super soaker.  

Quote from NatyaRudh >>

If a paladin or cleric uses Ceremony to Bless the water inside the decanter, do they have a limitless source of Holy Water? Additionally, if so... would the 'Geyser' command inflict Radiant Damage to Undead and Fiends? Perhaps as a 2nd die?

I would say no. As the item's description implies it creates water out of nothing. The ceremony spell specifies "a vial of water" which is only so much water. This is infinite water, it's way more than a vial. NTM, the decanter isn't a vial, semantics I know, but semantics are important in RAW.

In regards to my first argument, the item says "This stoppered flask sloshes when shaken, as if it contains water. The decanter weighs 2 pounds." And then it says: "You can use an action to remove the stopper and speak one of three command words, whereupon an amount of fresh water or salt water (your choice) pours out of the flask. The water stops pouring out at the start of your next turn." Now, if I were to remove the stopper, and turn it upside down, what would happen? Judging by the language used, it would emit the sound of sloshing water, and that's it. 

Now, as a DM, you should feel free to rule differently, but I wouldn't do that, since the party now has infinite holy water. Via pages 150 and 153 in the phb we find out that a flask of holy water goes for around 25gp (or 12.5gp if price is halved upon selling), and that a flask contains 1 pint of liquid.... This item can produce 30 gallons a round. If 1 gallon is 8 pints... 6 seconds of usage should produce an amount of holy water worth either 6000gp (or 3000gp if price is halved). Of course this assumes that a single person using this item takes 6 seconds each time, which is kind of silly. But so is the aforementioned amount of gold.

Or how I would like to call it, "Strahd's Worst Nightmare."

It's a great item, with a few days time and the help of 6 (I think) soldiers, I'm trying to build a massive moat around a city to help protect it from two different army's and a death zamboni. I don't know if it'll work against the army as that is next session, but it proves that endless water can help.

I wonder if a DM would allow a locathah character to use this to  bypass the whole drowning on land thing

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.