Go inside the Amatsu Sushi Bar (amatsu 217, 116) and talk to the Sushi Master (ama_in01 162, 17). Choose the 3rd option.
He will ask you to bring him certain items and when you return you will be rewarded with some Raw Fish or Sushi:

10 Candy for 15 Raw Fish
10 Conch for 30 Sushi
10 Fish Tail for 15 Raw Fish and 20 Sushi
10 Crab Shell for 20 Sushi
1 China for 20 Sushi
100 Mermaid's Heart for 50 Sushi and 50 Raw Fish