If this is an isolated sentence, I don't know what to suggest, miimi2! If the sentence is in a context, the context will tell you which one to choose. You really cannot answer the question unless you know where the passengers are (on the shore or the boat) and what they want to do (get to the shore or get on the boat)!
If you have absolutely no context, then I'd say StrangeAttitude's answer is the right one: many native speakers would assume that you were trying to get aboard before the ship before it left. This was also your first choice! So, that's what I'd go with.
In the old days of fancy-schmancy cruise liners, the call would go out shortly before the ship made way "All ashore whose going ashore". This was a signal for non-passengers to leave… well-wishers or personal staff.
If the ship is leaving in 15 minutes then I would dare to suggest that there isn't enough time to leave the ship, do anything worthwhile and get back aboard.
As you must choose one, I'd go with 'aboard' as it is more likely to be correct.