This question speaks of watery sphere as a replacement for feather fall because of this line:
If [the sphere] moves over a pit, a cliff, or other drop-off, it safely descends until it is hovering 10 feet above the ground. Any creature restrained by the sphere moves with it.
When I first read this line, I interpreted that as a way to prevent players from moving the big bad over a 400ft cliff and dropping him. The answer to the question above, however, mentions concentration being a potential reason why the sphere wouldn't reach the bottom.
If you could drop concentration willingly to drop the big bad 400ft, that seems to disprove my reading of it, but if you couldn't, that seems to imply that the sphere moves instantaneously, with no room to lose concentration (while still being "safe").
How fast does the watery sphere move while descending safely, and is releasing concentration before it does possible?
Best Answer
RAW, Up to the DM
Watery sphere does not specify how fast the sphere descends, and although there are rules for how fast a creature falls in freefall that other answers have noted, I would not describe that pace of descent as "safe" - and those rules do not strictly apply to magical effects anyway.
How I would rule it: 60 feet/round, following other spells with "safe" descent
The spell Feather Fall states:
The spell Earthbind states:
This implies to me that 60 feet/round is a "safe" rate of descent in the game - thus I would use it as the rate that a Watery Sphere descends.
Regardless, you can drop concentration at any time
So you could move the big bad trapped in your Watery Sphere off the cliff, then immediately drop concentration before it starts descending.