The title says it all; You can't empty water buckets into existing water. They don't empty if you have something in your offhand or not, right-clicking underwater blocks to empty the bucket only plays the bucket emptying noise.

   Coincidentally, this sometimes causes a bug in which the bucket texture sometimes doesn't match the liquids it stores, if any. For example, an empty bucket will actually place water when right clicked with, and a water bucket will update to an empty texture when used.

Edit: To reproduce (Not reliable, seems to randomly do it when using a water bucket)
1. Right-click water source blocks many, many times.

2. Attempt to place water on dry land.