I have no mods and I can only pour water in certain places at certain times, and other times it does not work. Any ideas what's going on?
This is a known bug, there is no fix at the moment.
Things to try if your water vanishes when you place it.
Move away from the spawn point. Water that is scooped from near the spawn point may be bugged. Water that is placed near the spawn point may be bugged. Spawn point = your starting location with no bed AND/OR your bed. Far = at least 33 blocks away horizontally.
If you're playing with multiple people - get the primary person (the one that owns the savegame) to place the water. Second split-screen player may be bugged.
Water seems to be working better since the June 15th update.