on board / aboard

[venice_queen]

"Please, call me or send me an email if others join your party to take this tour, even if you are on board"

"Please, call me or send me an email if others join your party to take this tour, even if you are aboard the cruise ship"

How can I know which one to use? Do these words have a different meaning?
Thank you!

 

[JJohnson]

Either one is correct. I prefer the "aboard the ship", and, whichever you use, keep the phase "the ship" or "the cruise ship" just to keep the meaning clear.

"On board" is OK, but it could mean "on the ship" or "on the team and part of the group in spirit".

 

[Embonpoint]

I would say "aboard."

you might add the word "already" for clarity, as in, "even if you are already aboard the cruise ship."