One of the biggest myths surrounding breastfeeding is that there is a long list of foods that nursing mothers need to avoid. In reality, there are very few if any foods that all nursing mothers need to avoid. Certainly there are foods such as caffeine and alcohol that less is better than more, but for the most part, you are free to eat just about anything. This is just common sense nutrition for all of us - regardless of whether we are nursing.
Moderation is the key.
If a certain food that you eat seems to bother your baby, try leaving it off and see if that helps. This is especially important for foods that trigger allergic reactions to anyone in your family. Eat foods that have been properly prepared to minimize exposure to either you or your baby to disease-causing organisms. This applies to sushi - make sure that the sushi you eat is fresh and prepared by a reputable sushi chef. Probably the greater danger in a nursing mother eating sushi is that she might get ill and feel less like caring for her infant.