Hi, you will have a wonderful time -- I'm actually onboard the Golden Princess right now (we leave Ensenada to return to LA tonight). Be sure to take the onboard ukulele lessons and tell Dave from Elua (the instructor in the husband/wife duo) that Staci said hi!
In Lahaina you can walk to a uke store just beyond Hilo Hattie -- it is Lahaina Music. In Honolulu, you dock right at Aloha Tower Marketplace and they have the Hawaiian Ukulele Company, but you are super close to the Kamaka factory and if you have time for the tour I highly recommend it. There are a lot of uke shops in Waikiki but they are expensive -- you could take a bus to them from the pier though. Check out the uke store thread for ideas.
In Hilo, there is Hilo Guitars. It isn't really walkable from the ship but if you take a shuttle to town it is quite walkable and they have a nice selection. In Kauai the only walkable shop is not good, but if you can get a car, Larry's in Kapaa is great, as is the uke store in Koloa, and I think they have been having big sales at Scotty's in Kalaheo.
You will have a great time, enjoy!