Is ONE PIECE episode 38 filler?
Canon No, episode 38 of ONE PIECE is canon.
#38
Luffy in Big Trouble! Fishmen vs. the Luffy Pirates!
ルフィ大ピンチ！魚人ＶＳルフィ海賊団
Aired 23 Aug 2000
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.