There are many different types of sushi. As of now we are going to cover two types:
Maki sushi contains strips of fish or vegetables, and rice wrapped in dried seaweed (nori). For maki types, the correct Brocho is on the majority ingredient. E.g. If the majority ingredient is rice (which it usually is), the correct Brocho would be Borei Minei Mezonos.
If you cannot tell what the majority ingredient is, say a Borei Minei Mezonos on another Mezonos product, and Shehakol Niyeh Bedvoro on another Shehakol product [not water], with intent to exempt the Sushi. If no other product is available, pull out a piece of rice, say a Mezonos, and eat it. Then pull out a piece of fish, briefly put it down separately, and then say Shehakol, and eat it. You can then eat the Sushi.
Nigiri Sushi is an oblong mound of rice that the chef presses into a small rectangular box between the palms of the hands, which is then covered with fish [like the picture]. For nigiri types first say a Borei Minei Mezonos on the rice, and then a Shehakol Niyeh Bedvoro on the fish.
Source: Hakhel Bulletin (Volume 4, Number 1)