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.
Thank you for your report!
We're actually already tracking this issue in MC-181499, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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