Lucky you! The Diamond is a fabulous ship.
Here is my take on luaus! Don't, but if you must, the very best luau in Hawaii is "The Old Lahaina Luau" on Maui. It is an evening thing though and would probably not work for your schedule.
The tourist luau generally consists of food and a show. The food is interesting but not particularly wonderful, consisting of kalua pork (pork cooked in a pit), Lomi Lomi Salmon (raw salmon with a marinade of tomato and onion), Huli Huli Chicken or something similar (Chicken with a sweet/sour sauce), and of course Poi to accompany it all (very few people who go to a luau to experience Hawaiian food like poi. A few do and I am one of them)! These staples are often accompanied by fried rice, some kind of fish, and macaroni salad. Dessert is generally a coconut gelatin style pudding called Haupia. It is delicious. Usually a couple of drink coupons are thrown into the mix.
The Old Lahaina Luau's show is strictly Hawaiian chants and dance. You will not see dancing from any other of the Island groups. In other words no Maori dancing, no Fijian dancers, no fire dancer etc.
If you want to see a really good show and enjoy a taste of some of the Luau food I would suggest that you visit the Polynesian Cultural Center on Oahu. As well as the luau and show you will have a chance to visit "villages" from 7 different island groups and learn a little about each of their cultures. Go the whole distance and purchase one of their Ambassador packages that includes a guided tour. The guided tour will get you to each of the "villages" as their show starts and you will get a lot more out of your day. Having done the PCC on my own and on a tour I know that the tour is the only way to go...especially if you are a first time visitor. Since OLL is owned and run by the LDS you will not find any alcoholic beverages on the property...so no free drink coupons, just fruit punch :grin !
Aloha!