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