Karpas is meant to symbolize the bitterness and salty tears we cried during our ensalvement during egypt. 

The way this can relate to One Piece is that on the island of Onigashima Luffy, Law, and Kid, were fighting two emporers of the sea at the same time. (emporers of the sea are some of the most powerful people in the One Piece world) After defeating one emporer, the other named Kaidou knocked luffy off the island (which is floating in the sky) and he fell into the ocean (which is saltwater). In case you didn't know, Luffy's abilities get taken away under water and is unable to do anything when submerged. Luckily, someone who he has an alliance with, Law, saves him. Luffy is taken out of the salty sea water and saved. He returns to the top of onigashima and continues fighting. 

Dipping a potato in salty water is similar to this

haggadah Section: Karpas