Yeah, agreed. I guess we could do something like waterway=ditch (to describe what it is now) + abandoned:waterway=canal (what it used to be). I guess split the way and don't have ditch on the parts going through what is now ponds. The wiki page https://wiki.openstreetmap.org/wiki/Tag%3Awaterway%3Dderelict_canal basically suggests not to use waterway=derelict_canal because it's not very descriptive.

If someone wanted, they could also draw polygons covering where water actually is and tag natural=water, though that's a fair bit more work.