Wiktionary
dull as ditchwater
a. (context simile English) (alternative form of dull as dishwater English)
Usage examples of "dull as ditchwater".
Danny had left her saying she was as dull as ditchwater and they had nothing to talk about.
Well, that was all right, nice lad, son of Dillmouth's leading solicitor, but frankly, dull as ditchwater.