Rego Aquam doesn't have any specific base guidelines for teleporting water as far as I'm aware (not in ArM5 CRB, at least - maybe in a splat), but we can fake it. Obviously we need to enchant an item - but first, we have to figure out the effect.
"Transport the target instantly to a place to which you have an Arcane Connection" is RcCo 35. It seems reasonable doing the same for a base sum of water is ReAq 35.
The range is (obviously) Arcane Connection, for +4 magnitudes. Rough. Level 55 is rough.
The base individual for Aquam is "a pool of water about five paces (fifteen feet) across, and two paces deep at the center." Processed and such are worse, but we're just looking for water. That's more than enough to fill a decanter, so we don't need to worry about bumping individual at all. Arguably, you might need to bump to Part (to ensure you don't get too much) but that would be a bit weird that you could do the same with a pool of corrosive liquids at a lower magnitude. As such, I'm inclined to leave the target at Individual. Speak with your storyguide (unless someone can come up with cites.)
The duration for a teleportation effect is momentary, so no worries here.
So we're looking at ReAq 55 for out base. This would be a problem for a formulaic spell, but it's fine for an enchanted item, as there's no other reason that this would need to be a ritual.
You have the option of including a trigger here - for example, an InAq 2 base effect to "get an Image of water within range", bumped 1 mag to touch. The trigger would detect when the container was empty, and automatically trigger the core (ReAq 55) effect. It'd probably be simplest to make it continuous by making duration Sun (+2, for spell level 5 after all mods), giving it 2 uses a day (+1 level), and an environmental trigger (+3 levels.) In all, if you want it to automatically refill, rather than requiring use of their trigger action, you need to include 9 levels of trigger and increase the base by +3, for linked trigger.
Right now, we're at 67 levels of effect, but we're still missing uses per day. If you think you won't need more than 50 uses, you can skimp by with +6 for effect frequency, but I'd spring for Unlimited, raising your final, total levels to 77. You have to separately instill ReAq 68 and InAq 9. You'll also have to incorporate an arcane connection - probably a fixed one - into the construction of the item.
77 levels of effects is pretty tough to squeeze in, especially with it being so dominated by one effect. However, it is pretty cool. Also, if you tie it to the ocean, you have an unbounded sum of water to dump out. Beat that, CrAq!
EDIT: It appears I erroneously missed some changes to the ruleset. I don't have access to the books in question, but to my understanding, the primary effect ends up 2 magnitudes lower. Doesn't help a lot, unless I got something wrong. If I did get something wrong, please let me know!