My first ever! Well, first ever on purpose; I put the Robe of Deeppockets up last week.
I have always had a special fondness for magic items like Daern's Instant Fortress or the Bag og Holding; useful but not combat-oriented. So I will try to add a new useful item each week!
Magic item; Everfull Flask.This is a small metal pint flask designed to be slipped into a pocket [i.e., a hip flask] with a cork and metal stopper attached to the bottle with a chain. It is typically found full of resh water. At any time the flask is not full and the stopper is in place after 5 minutes the flask will refill itself with fresh, clean water that is pleasantly cool. Without the stopper in place the flask will not refill itself.
Other things may placed into the flask (oil, potions, etc.) but the flask will not refill itself until after the other liquid is removed. The flask makes all saving throws as hard metal.
The Everfull Flask is meant to be a 'nice to have' magic item about utility rather than effect. It only produces a pint of water at a time, the gap between pints is at least 5 minutes, and it only refills with the cap on - you can't use it to put out large fires or flood a room. So why have one? Well, with one of these you never need worry about dying of thirst.
Your players aren't worried about their characters running out of water? Maybe you should fix that. See, water is heavy - 10 lbs. a gallon [OF COURSE I use Imperial measurements!]. And you need at least a gallon of water every day just for drinking - toss in cooking and hygiene and you need a total of 5 gallons a day. If your players are sending their characters into a dungeon they need at least 2 full wine skins each for 8 hours of walking and fighting [for historical reasons I set a wine skin's volume at a fifth of a gallon] through a dungeon. That's 4 lbs. of water. Oh, they get stuck or lost and are there overnight? Then they start taking penalties for thirst and dehydration.
Or they can take the 'Flask!