September is hurricane season in the Pacific. I have never sailed there but noticed a large amount of storms on the weather map last year. In fact alot more storms than in the Gulf or Atlantic. I was told that the Pacific usually has rougher seas than the Atlantic.
I have found that the cabins in the aft tend to have less movement than the foward. I have also found the lower decks and of course larger ships are calmer too.
RCCL - Enchantment of the Seas - Eastern
Carnival - Fascination X2 - Southern
Carnival - Tropicale - Western
Celebrity - Millennium - Eastern
Celebrity - Century - Western
Princess - Caribbean - Eastern
We didn't go that time of the year to Hawaii, but I just want to say, "hopefully" you will enjoy your trip. We went in January (Hawaii is known for little rain at a time.), and it decided to "rain on our parade." When I say rain, I mean gushers. If the wind wasn't blowing where you could not get in/out of the cruise doors, unless someone pulled on the other side, or when we were on land, we were walking in 2 - 3" of rain.
Look up the word Pacific in your dictionary. Even the word means peaceful and calm. The name came from early sea explorers who found it remarkably smooth and tranquil once out to sea. You should have no worries. Enjoy your cruise!
I don't know if you are referring to the Serenade of the Seas or another ship, but a little over a week ago, we just got off. To say the least the ship was very smooth. The only time I noticed (I think I was the only one as I have a few problems) a little movement was a dinner one night, but it only lasted about 10 minutes.
We were in the Crown & Anchor area, and a question came up about the movement of the ship, asking the captain. He said this ship was very well balanced at the bottom (I could tell), so there would be very little or no movement that a person could detect.