Is ONE PIECE episode 691 filler?
Canon No, episode 691 of ONE PIECE is canon.
#691
The Second Samurai – Evening Shower Kanjuro Appears
二人目の侍 夕立ちカン十郎登場
Aired 3 May 2015
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.