In a large bowl, combine fish, shrimp and juice. Leave to marinate for 20 minutes. Drain the water except for 1/2 cup. Add remaining ingredients, season with salt and pepper to taste and stir. Refrigerate for 20 minutes.
For the sauce. In a food processor, combine cilantro, lime juice, salt and pepper until finely crumbled. Add mayonnaise and puree until smooth. Cool in the refrigerator.
For filing. Apply a thin layer of mayonnaise with herbs to the tortillas. Top with ceviche and sliced avocado.