There are several synonyms for the phrase "on ship" which can be used to provide more variety in writing or speech. One common synonym is "aboard" which is often used to refer to being on any type of vessel. Another option is "onboard" which is similar to aboard but specifically refers to being inside the ship or aircraft. A third synonym is "afloat" which can be used to describe being on any floating object such as a boat or ship. Finally, "at sea" can be used to indicate being on a ship that is far from land and actively underway. These synonyms can be useful for writers and speakers looking to add a bit more descriptive flair or variety to their language.