"Cooking" with acid (ceviche)
Ceviche is not exactly "cooked", but the acid causes the proteins to become denatured in a similar way. It may not kill all bacteria and parasites as effectively as cooking (with heat), so like sdg said, it's safest to use food that you would eat raw.

"Sushi grade" can refer to the fat content of the fish (like the USDA grades for beef marbling) rather than an assurance of its safety, so don't rely on that to pick out ingredients. Saltwater fish should be frozen at sea both for freshness and to kill parasites, and I prefer to get individually quick frozen (IQF) shrimp both for their freshness and convenience. Saltwater fish at your local supermarket has been previously frozen, so you're usually better off just buying frozen fish and thawing it yourself, unless you're really pressed for time.

Freshwater fish should not be used in a raw preparation, including ceviche.