How do I fill the barrel with water?

i dunno, but i think in the new update barrels will be fillable

read somewhere that at the latest in august, so it should be near

Open Barrels, water troughs, etc. aren't fillable yet.

The closed lid barrel just works the same way as a chest.

With the upcoming update, open containers (the open barrel or the trough) will automatically fill with water when placed outdoors during rain

Water bucket filling from lakes and spilling to make ponds as creative already allows spilling water?

Infinite water sources so we can make mountain lakes, waterfalls sourced by downhill streams replenished with hidden springs with a source fill rate? Just set the seas to never rise from too much flow (though maybe someone wants to slowly play out the shift to water world), but overflowing lakes and flooding creeks from changing the rate seasonally or rains would be fun.