The weather in Hawaii in September is not much different that the weather in Hawaii most any other time of the year, generally sunny in the mid-80's. The only possible difference at that time of the year is that sometimes the trade winds stop and the humidity may get a little uncomfortable. Also, the weather may vary from place to place on the islands, with one side of the mountain (leeward) being in sun while the other side of the mountain (windward) has some showers.
September and October are great. November and December starts to get in the rainy season. Go to weather.com and look up weather for Honolulu and you can see a month by month chart of temps and rain fall totals.
I went to Hawaii from Vancouver in September/October 2001. The best I can remember we were pretty much fog-covered for the first 2-3 days at sea and the weather was on the cool side. That particular sailing had 5 days at sea. We had prearranged early excursions for each port - we were glad we did. By the time each excursion concluded we were pretty much ready to head back to the ship due to the heat. Up side -- we didn't spend a lot of time shopping. We would look around the area near the port and head back to our home away from home and sit out on the deck and just admire the scenery (in most ports).
I loved it - I'm going from Ensenada to Hawaii in March on Infinity.