Sandwiches

"Water all around" is to clue that the puzzle is about islands.  The Sandwich Islands, named in honor of the Earl of Sandwich, are now better known as the Hawaiian Islands and there are indeed 18 major islands in the Hawaiian archipelago (many sources say 19, but I have yet to find one that lists the 19th.)

There are 12 distinct sandwich ingredients in the puzzle (excluding bread) and 12 letters in the Hawaiian alphabet, thus this is a simple Hawaiian substitution cipher to name the 18 islands.  Sandwiches 6 and 18 are a good place to start because of the repeated turkey and tomato, they must be Mokupapapa and Papa'apoho respectively.

As you get into it you'll realize that the sandwiches do not quite match the island names exactly.  This is because each of them has one or occasionally two extra letters added.  The cipher comes out as this:

     KANEMIHLOHAI               HAUWAII                      PMUHAHONU
     UHOLOIKAUAUA      NALUHKAKUALA            MOKMUPAPAPA
            LANAUI                     PIHEMANUN                       OUAKHU
            KAUUAI                  KAHONOLAWE                     NIIHUAU
MOKUKMANAMANA        MOULOKAAI                        KAPUO
      MOUKUMANA                    MAAUI                        APAPAAPOHO

The extra letters taken in order (left to right, top to bottom) spell HUMUHUMUNUKUNUKUAPUA'A, which is the state fish of Hawaii.