abridged: speak the command word "Geyser" produces 30 gallons of water that gushes forth in a geyser 30 feet long and 1 foot wide. As a bonus action while holding the decanter, you can aim the geyser at a creature you can see within 30 feet of you. The water stops pouring out at the start of your next turn.

Doesn't seem like the decanter has thrust. To design something that thrusts would require your character to have a deep understanding of D&D physics (or maybe just some deadly experimentation?!)

Trying to mix mundane physics and universe book rules (i.e. peasant railgun) in a D&D universe sounds like a dangerous pastime for a character.

Cue the old SF story where Hitler wants to control a portal to other planets. Version 1 destroyed when other end moved into a red sun creating a radioactive lance, Version 2 destroyed when other end put into an ocean creating high pressure hose.

Do you want your character to magically refine Plutonium, Madame Curie?