When the first Polynesians arrived in the Hawaiian Islands, they found nothing to eat. The Islands themselves, being very new geologically and very isolated had little in the way of indigenous life. The Polynesians brought with them, pigs, dogs, chickens, taro and sweet potatoes. These plus whatever seafood they could catch became the staple of life. After James Cook arrived in Hawaii, a steady stream of newcomers started arriving, bringing with them plants and animals from their native lands. Chief among these were missionaries from New England who brought cows, horses, sheep, and goats as well as a wide variety of new plants. This introduction of new foods and traditions changed the Hawaiian diet greatly. As American business began to move in and develop large plantations, new cash crops, like sugar cane and pineapples were introduced. To man the plantations, immigrants were brought to Hawaii as laborers from places like China, Japan, Okinawa, Korea, Puerto Rico and the Philippines. Each new wave in immigrants brought a new cooking tradition with them.
What developed from all this is known as "Local Food". Local Food is an East-West Pacific mix created in the 1920s and 30s by lunch wagon cooks and small stores and restaurants. Typical Local Food meals are lunch plates which consist of rice and meat covered in gravy, a raw fish dish called Poke served with either salt and seaweed or chili peppers, sesame oil or soy sauce. Also popular is Teriyaki, which is used to prepare all kinds of meat.
As you can see, Hawaiian food is a unique cuisine created from a much diversified background. It is common to see restaurants that serve American, Chinese, Korean and Local Food. And next door you'll find a place that serves Barbecue. A particular favorite in the Islands is grilled fish. What kind of fish you ask? How about, Aku, Ahi and Albacore (Tuna); Marlins; Opakapaka, Kalekale, Onaga, Uku and Taape (Snappers); Hapuu (Grouper); Ulua, Akule and Opelu (Jacks); Ono; Squid and Octopus; as well as all the shellfish like Opihi, Wana (Sea Urchins), Black Crab, Lobsters, Shrimp and Prawns. Now, about that Luau. Despite what Trader Vic or Don Ho might lead you to believe that Luau is a very rare festival in Hawaii. The sheer cost and effort of purchasing a pig, storing it until cooking time, preparing the Imu (traditional Luau cooking pit), preparing the loads of raw fish and additional dishes, is quite the undertaking, so several recipe variations have popped up to those of the formal Luau.
In recent years a new tradition of cooking has evolved. Called Hawaiian Regional Cuisine, chefs have begun to put away the Spam (Hawaii is the largest per capita consumer of Spam) and have started to develop a new flavor. Mixing the cooking traditions of the Orient with that of the United States and using local products. What you get are a lot of seafood dishes (grilled of course) mixed with oriental style sauces, made from things like fresh guava, taro, mango, etc. From the looks of things I think Hawaii will start exporting many new dishes back to the mainland.