Splash! I'd suspected the other bird was a Paradise Flycatcher (サンコウチョウ）[Terpsiphone atrocaudata] .

There is a splash but it's hard to see what makes it. The bird concerned either doesn't mind that the gray-eared bulbuls (ヒヨドリ）are there, or regards them as a good warning system.
Bulbuls too don't stand in the water to bathe but jump in and out without stopping.
Add your suggestions in the comments please.
PS Today I found a better view in a bit of new footage.