mentioned that they like one piece:

• s.coups
• jeonghan
• joshua
• jun
• hoshi
• wonwoo
• woozi
• dk
• seungkwan
• vernon

played a free association relay on a vlive and said one piece for "manga" which has to mean something:

• mingyu

never mentioned but assuming they do:

• the8
• dino

"we love that one piece lovin boy group!"

• their album, going seventeen, was inspired by one piece
  • in one piece, there's a ship called "going merry" and on the ship, the one piece characters declared the dreams they would be chasing. for the album, going seventeen, their concept was "embarking on a voyage of dreams" and their photos had a pirate/sailor concept (ocean, boat-building, etc)
• woozi was asked which characters joshua, jeonghan, and vernon would be and he listed one piece characters