When it comes to describing being on a ship, there are several synonyms that can be used instead of "aboard a ship". For instance, "on a vessel", "on a boat", "on a craft" or "on a seafaring vessel". "On the high seas" or "at sea" are also options that could be used to describe being on a ship. Moreover, terms like "on deck", "in steerage" or "below deck" can be used to describe different locations on board a ship. In summary, there are many synonyms that can be used to describe being on a ship, depending on the context and the location on board.