Low down midships is the best place for not so good travellers. The 'back end' is better than up front. The higher the deck the more movement you will experience. A cruise ship is the one place that charges more for the 'worst if there are rough seas' cabins ! I went on Grand Princess round Britain. She is pretty stable but I am one of those who love being 'rocked' to sleep even in my midship cabin.
This is my worry too . I have had to book one at the front and I'm not wonderfull going through the bay so I'm not looking forward to that bit. People who were ill on our cruise in august said the travel pills from the shop on board were very good. We heard that to go and see the doctor and get the jab was $60. We tried to avoid this as didn't want to spend that much unless no other option.
I too am a very bad 'sailor'. I have travelled on two other large Princess ships and can assure you that if you feel ill just go to the medical centre. The service is excellent. I was given a check up and then an injection and after a two hour 'snooze' felt on top of the world. I even managed to go whale watching in a small vessel mid holiday - Unheard of for me. Well worth the small additional cost and the medical teams on boards Princess ships are terrific.