Sushi is great. I love eel and tuna. I was bold and tried sea urchin one time. I looked like chopped liver (if you've ever seen it after going through a blender). It had the consistency of--hmm--like pudding, or blended liver..not very good texture, I'll tell you that. It tasted good though! Anyway, I love sushi, but it is not a very good idea to eat tuna (or some other fish for that matter) very often because of mercury levels.
Here is a good place to look to see how much fish is ok. You enter your weight, choose your fish and how much you'll eat. It only takes 4 oz (at my 120 lbs) to reach the 100% mercury exposure. Its insane how much mercury is in some fish.