Who said anything about one-time use? I had in mind a one-time-use item (an "oh shit, we're in a tight spot and need a big emergency 'abort' button, even if even that option is risky" sort of item) when I gave it to them, but I definitely wouldn't have made what they came up with a one-time-use item. (Unless they rolled relatively low on the process of engineering the gun in the first place, in which case I might have been like "yeah, you've got something that you think will probably work like maybe once, but you're not confident it would work more than like once or twice without failing catastrophically".) Probably a roughly infinite number of uses. (Unless they, like, propped it on a rock and left it "on" over a long rest or whatever.)

I figured they had an "ocean" worth of water. (Well, as I mentioned in another comment in this thread, I think it would be reasonable to make it well less than a literal ocean, but still, a fuckton.) And they'd be firing in like less-than-6-second bursts. They'd have to shoot that thing a lot of times to exhaust its water stores enough to start losing pressure.

And, just to be clear, what I had in mind was a very narrow and high-pressure stream that would do a shit-ton of... I dunno, slashing(?)... damage to enemies. Maybe even cutting holes through one guy and doing damage to the guy behind that guy if the enemies happened to be lined up (or if the PC wanted to orchestrate that). But such a powerful weapon would need drawbacks, I'd think. Like maybe a minus to the dex roll to hit? Or even disadvantage on the roll to hit. Plus the whole pushes-the-shooter-back thing would be interesting in some situations. (Could even be used strategically. Even if you missed, maybe this would avoid attacks of opportunity because you didn't move out of melee range. You were pushed by the force of the water? Maybe?)

(Also, to be fair, I'm thinking in 5e rules here and the original campaign in which I introduced this item was PF1e. Anyway.)

Anyway, what I'm getting at is that I never had one-use in mind for what they came up with. And it sounds like you might have thought I was saying its only effect would be to push the shooter back or something. Not at all what I meant.