My thoughts - hopefully relevant even though not exactly on legality.
Had an 8 day walk pre-2003 fires, in which our last night was at the Opera House. Had been advised by a friend of SMA worker that he & they regularly used the said tunnel. "When you get there, you'll see what i mean - no problems! Only ankle deep" I was still unsure, but at that time the 'goat track' traversing Watson's Crags still apparently existed. (Google on this - some say no sign of it now, which I would strongly expect is the case.)
We had good weather for 6 days until White's River hut - sat out a day of rain / whiteout on the Rolling Ground. 2 nights later we were at the Opera House. Found the tunnel with water gushing into it from pipes. Huge stopper waves that would have been at least up to my waist... or neck...
Don't know how long it took to get like that, but like i said, it'd been only 1 day of rain, a couple of days earlier. This was in summer.
Fortunately found the goat track in the end, hiding well under all the ferns etc
Hope this helps!