Is water useful?
Maybe the intended question was more about how
"water" might be useful (or not useful)?
I can see how pain can help in some things, but not in others. In terms of sensing minute details in the body, pain usually seems to wash out a lot of the signal. Also, I tend to think that if pain is present, something isn't quite right on my part (as uke); it's a sign I'm not fluid or otherwise engaged enough in some part(s) of my overall structure.
That said, though, I also pay close attention to the "shape" of my pain and have had the sense of feeling around "in" it to see what's going on and then figuring out something on how to respond as a result.
As a slight side-note I'd just like to add how I was always surprised to take ukemi for Sensei Barrish when I had an injury (i.e. mild strained/sprained joints, etc.) because I almost never felt any pain. If anything I usually felt a little stronger in the offending joint.