(Photo from Ezu Restaurant) It is true that the majority of sushi is fish - tuna, crab, salmon, shrimp, yellowtail and fresh water eel. However, there are plenty of non-fish sushi out there event a vegetarian person can have a complete meal without ever eating one fish sushi.
Here is a list of non-fish sushi.
Traditional Japanese Non-Fish Sushi Kappa - Cucumber roll. Japanese maki standard. Refreshing, so it's good to eat between fish nigiri to cleanse your palate.