https://youtu.be/akxqOJ1kzKA

place a water source anywhere.

hold an empty bottle in your main hand

flick your camera onto the water source, right click, and flick it back up as fast as possible.

Occasionally, but not all the time, the water bottle will appear filled, show as filled, and be drinkable as if it was filled.
However you will not be able to drink it.
You can hold right click as long as you want and the bottle will not empty nor will the drinking animation and slow effect stop.

Scrolling to a different slot and right clicking (an item) will refresh the water bottle and it will return to the empty state.