When I jumped off a bridge at build height aiming for a water pool, I fell next to a (user-made) waterfall and a tree. I didn't touch the tree or the waterfall, and I thought that maybe I touched the waterfall right before I clipped the edge of the tree, but I didn't.

Right before I touched the water pool, I saw bubbles underwater for a split second when there was only dirt, stone, and coal below the water (not soul sand or magma blocks that emit bubble columns). I assume this is the particle effect for taking damage underwater, since when I fell into the pool, I took half a heart of damage. It was presumably fall damage, but I didn't touch anything that seemed to effect fall damage.

Summary:

Pool was one block deep

Fell from Y level 256 (player position, not block position)

Fell into pool at Y level 68 (player position), a 188 block fall

Took half a heart (1 HP) from falling into water 

Damage was taken underwater when touching the underwater block

Attachment: (exceeded 10 MB) Video here on YouTube, is in slow motion