Consider this mod experimental. Feedback and bug reports are appreciated!

Right now I'm using the Minecraft sprites, so take care not to get confused with buckets from other mods such as Gregtorio.

Buckets

Craft a Bucket by placing 3 Iron plates in a V-shape. You will not need a crafting table.

Buckets can hold a variable amount of almost any fluid, up to 5,000 units (and this increases with quality). They do not stack when filled.
They can be filled/emptied into buildings by hand, like a literal pipette. Good for when you just want to get some fluid from A to B, or you want to rescue the liquid neutronium you made by accident.

They are strictly manual; they can't be manipulated by machines. Rockets can't carry filled buckets either. (It would make a huge mess!)

Press the "Use Bucket" keybind (default is G) to access the functions of the bucket.

Interact with tiles:

• Fill the bucket by hovering over water (or any other liquid-giving tile) and pressing "Use Bucket".
• Empty the bucket (destroying the contents) by hovering over a normal tile and "Use Bucket".
• Empty a bucket in your inventory by hovering over it and "Use Bucket".

Interact with buildings:

• Fill the bucket from a building's GUI by hovering over the fluid icon and pressing "Use Bucket".
• Empty the bucket into the building GUI by not hovering the fluid icon and pressing "Use Bucket".
• You can also Fill/Empty the bucket by hovering over the entity in the world.

Barrels

"Wait what's the point of all this? Barrels are how you carry fluids in items!"

Barrels can't carry everything, y'know.

All of the "Use Bucket" functions also work to manipulate barrels. (In fact, you can just turn off Buckets entirely and use barrels to move fluids around.)

You can also transfer fluid between buckets and barrels in your inventory.