Citizens won't use buckets already containing water to fill ponds. The fill pond job is hardcoded to look for an empty bucket and then fill that from a water source (which custom workshops do not count for) before dumping the water in the pond zone. Toady only very recently implemented a logic check on this job to dump out partially filled buckets of water to then fill with water for the pond, as previously partially filled buckets of water required DFHack intervention to be used again.

I don't think barrels of water will be seen as a water source for drinking, as the water material (which is hardcoded) does not have either ALCOHOL_PLANT nor ALCOHOL_CREATURE. Drinking water from a well/water source is a different job that replaces the seek drink job if no valid drinks are found. I'm pretty sure that drinking from a water source won't see custom workshops nor already full containers as valid sources of water, though this would require testing to confirm.

'not much use' how about QoL when flling huge ponds? or leaving a barrel of waterfor human/elf prisoners so they dont alcohol poison

This is brilliant trying it now

Makes sense. Great work on the mod.

Jugs have a capacity of 15 units, but it's never utilized. The most liquid that vanilla places in a jug is 5 units of olive oil, and that requires a high level herbalist to obtain a stack of 5 olives (quite rare). Everything else that uses jugs (honey, royal jelly, and all other oil pressing reactions) only create 1 unit of liquid in the jug.

The reactions aren't intended to always create full containers anyways, they're options for mods to utilize various quantities of water easily. A small scale reaction can use a jug, a medium scale reaction can use a bucket, and a large scale reaction can use a barrel. Reactions have a lot of issues with liquid reagents, so it's safer to be able to specify the container type as a method of determining the quantity of liquid called for.

Large pots are already accounted for in the barrel reaction.

Jugs should hold more water. They hold 15 units of liquid in other circumstances, flasks and water skins hold three units of drinkable liquid. There's also a Large Pot item that's comparable to a barrel.

@CondoR The workshop can be built and work without any access to water. There's currently not a function in the game for water like there is for magma-fueled workshops. Vanilla only uses water in buckets for two things: filling ponds and hospital drinking. I know the water from this can't be used for filling ponds, because that job is programmed to start with an empty bucket. I don't know if this can be used for providing water to a hospital. Water in jugs and barrels isn't possible in vanilla.

Finally, I can mod in my fantasy of the ultimate bucket brigade

Can this well be used on maps with no water sources at all? Is water filled into barrels, jugs, buckets used like vanilla?