General cauldron info
Cauldrons are blocks that hold water. They are crafted from 7 iron ingots and they always spawn empty in a witch hut. To fill a cauldron with water, right-click on the cauldron with a water bucket. A cauldron also has a chance to fill up with water when rained upon.
Cauldron uses and mechanics
Cauldron cannot be used to fill an empty bucket, and water bottles cannot be used to refill a cauldron, but a water bucket can fill a cauldron to it’s fullest. A cauldron can be filled from a water bucket in the Nether. You can fill up to 3 water buckets from a full cauldron.
A cauldron can act as a power source for a redstone comparator. With a cauldron behind it (possibly separated by an unpowered solid block), a comparator will output a signal strength proportional to how full the cauldron is: 0 for empty, 1 for one-third full, 2 for two-thirds full, and 3 for completely full.
When a cauldron is destroyed (dropping itself as an item), any water inside it will be lost, so it’s better to first pickup the water and then destroy the cauldron.
My opinion on cauldrons
They are very useful blocks. This way you can bring and store water in The Nether. Cauldrons are often used for secret passages, because if you are pushed by a piston while standing in the front of a cauldron, you will fall through it if there is a free space under the cauldron.