Is ONE PIECE episode 524 filler?
Canon No, episode 524 of ONE PIECE is canon.
#524
Deadly Combat under the Sea! The Demon of the Ocean Strikes!
海中の死闘 現れた大海原の悪魔
Aired 20 Nov 2011
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.