Well about this, we all think different,
I can only tell by my experience;
You cannot consider it a Pendulum, because the pendulum is fixed at the top, the ship oscillate around it's center (center of mass)not the top,
Because the ship is not fixed at any point.
The best guess will be to book a cabin on the center of the ship, but that will depend on its design and because it's center of mass will change depending on the distribution of the mass of the ship,It does not have to be exactly on the center...
I actually ask two other professors of physics just to check my point, because of last time and they also think the same way as I do..
The other day Travel channel on their program
"Cruise do and dont's"
Gave the same recomendation, as I do, base on their experience in order to avoid sea sick...