Dungeons and Dragons - Memes and Comics

First, my plan was definitely to make my version of the item not literally produce an ocean's worth of water. Just enough to make a tidal wave big enough to wash everyone out of roughly the biggest dungeon I was likely to put in that campaign. (Hell, it would also be reasonable that part of the force propelling the "tidal wave" would be magic, not actually just the force of a huge quantity of rushing water.) And, yes, there would be dex saves involved to avoid taking damage from bashing into doorjams and such.

Second, it is magic, right? If it was under an ocean's worth of pressure, it wouldn't remain intact even if undisturbed. It'd easily burst. Who knows how it actually works. One fairly obvious choice would be that it contains a pocket dimension full of water that empties into the user's current plane when broken. (Or maybe it draws water from the elemental plane of water.) Maybe the water visible in the container is strictly cosmetic. Maybe drilling a hole would make a tiny puncture in the boundary between the driller's plane and the pocket(/elemental) dimension. Maybe that planar puncture would be present on the outer surface of the bottle rather than inside where it would put a lot of mechanical force on the structure of the bottle.

Third, it would be totally valid for the process of constructing a gun out of it to require quite the skill challenge. Just be like "ok, who wants to contribute to the process of making this gun?" And everyone who wants to pitch in, great. If when they've all talked about their plans, if thr process they've come up with doesn't seem like it could reasonably succeed, make it catastrophically fail. Otherwise, make 'em all roll something. Setting the DC high would be totally reasonable. And if they fail, it blows up while they're attempting to build the gun. If it's close, maybe they get a gun that works, except that they have to roll a D20 every time they use it and on the third 1, it explodes. (Could be even more wiley. Secretly roll the D20 every long rest after they've used it during the "day". If they happen to examine the gun give an account of how much damage is present based on how many 1s have come up. If one, maybe say "there's a little bit of chipping near the drill hole." If two, "a crack runs a good couple of inches from the drill hole down the side of the ampule." If they don't actually examine the gun, they might have no idea what's coming until... On the third 1, it explodes while in the PC's inventory during the long rest, interrupting the rest and washing them far from where they were. Maybe even separating the party and losing some of their items in the process. There are so many possibilities. What if it was in a Bag of Holding at the time?)

Hell. If an enemy looks at the gun and recognizes the bottled ocean integrated into it, the enemy could use it against them. Maybe target it specifically with crossbow bolts. On a hit, it triggers the by-the-book effect.

I guess basically what I'm getting at is: rule of cool. I obviously didn't anticipate what the PC's had in mind for that item until they brought it up. But I would think a good GM would try to find a way to make it work.