HEY!
I travel only by cruise so, my insight. A) The gyms almost ALWAYS have cardio rooms. Look at the schedule and see when this room is free. Bring your lap top and have fun.

Cruise ships also have tonnes of public space rooms. If you are working out in the morning and the cardio room is unavailable, see if you can use the night club or some other room which has a designated dance floor to work out in. On the cruise ship we were just on, I used the night club in the morning because it wasn't being used.
c) Outer decks. There are ALWAYS top level walking decks or sports zones. When you get on the ship, use a map to locate these areas. There are usually quite a few...and hey, you'll get some sunshine when you work out.
If none of these ideas pan out, speak to the front desk or concierge on the cruise. Ask them for ideas. Failing that, and I don't know what kinda cruise you are on, you will most likely be in ports every day or every other day...... take your lap top with you on land and find a nice quiet park to work out in (or beach).
Cruise ships are REALLY accommodating.
Hope this helps!