The antonyms for "coming board" would be "departing board" or "leaving board." These terms indicate that one is leaving or going away, rather than arriving or entering. Other possible antonyms include "going out" or "moving away." "Coming board" is often used in the context of boarding a ship or aircraft, so an antonym could also be "disembarking" or "deplaning." Overall, antonyms for "coming board" communicate a sense of departure or separation, rather than the beginning of a journey or experience.