I really love both sushi and sashimi! These Japanese raw fish dishes provide great fish oils in a healthy manner. They are perfectly low carb.
If you want to go fully low carb, go for sashimi. Sashimi is, simply, the sushi without the rice. It's strips of raw fish. Every single sushi restaurant I've ever been in has offered sashimi. In fact, if you ask for sashimi they usually give you a much better piece of fish because they know that you aren't getting rice with it - the fish itself is going to be the sole way you judge the meal.
If you really want to have the rice with it, remember that this isn't a good whole grain, high fiber rice. It's ultra processed white rice with little nutrition :) It's also got about 45 net carbs per 1/2 cup. You easily get 1/2 cup of rice into you, depending on how much sushi you are eating and how they are making the sushi / rolls.
Soy sauce isn't carb free either. This is, of course, sauce made from soy beans, and beans in general are high carb. Soy sauce has 1.4 net carbs per 1 Tbsp. Most people I know soak up much more than 1 Tbsp when they're eating sushi :) Of course, it's still only 1.4g so if you're eating this with sashimi, a 1.4g intake across an entire meal is practically nothing. On the other hand, if you're gorging on sushi with rice, be sure to count this in to what you're eating.
While we're at it, most sushi restaurants provide radish - cut up into pretty designs and thin slices - as part of their sushi or sashimi meal. This is supposed to be eaten! This is to help your digestion. Radish is incredibly low carb - only 0.2g for an entire radish - so eat that knowing that it's very good for you both digestively and health-wise.
So to summarize, it's great to aim for the sashimi as the main part of your meal, and eat the radish as well. If you adore rolls, sure, indulge in one or two. Moderation is the key!
Lisa Shea's Library of Low Carb Books