here's 1 way:
1 pound fresh fish (snapper works well)
8 limes, juiced
2 tomatoes, diced
5 green onions, minced
2 stalks celery, sliced
1/2 green bell pepper, minced
1/2 cup chopped fresh parsley
freshly ground black pepper
1 1/2 tablespoons light olive oil
1/8 cup chopped fresh cilantro


1 Rinse fish and place in a medium sized bowl. Pour lime juice over the fish. The fish should be completely immersed in the lime juice. Chill the lime juice and fish all day or overnight until fish is opaque.
2 Empty 1/2 of the lime juice from the bowl. Add tomatoes, green onions, celery, green bell pepper, parsley, black pepper, olive oil, and cilantro to the mixture. Stir gently. Serve this dish in fancy glasses with a slice of lime hanging over the rim for effect. and Tostadas.

This works with shrimp and scallops too.