1. To prepare seviche, combine shrimp and 1 cup juice in a medium bowl; chill 1 hour. Add crab; toss gently to combine. Drain thoroughly; return shrimp and crab to bowl. Stir in 3 tablespoons onion and next 4 ingredients (through 1/4 teaspoon salt). Chill at least 30 minutes or up to 6 hours.
2. To prepare soup, combine peaches, 1/3 cup juice, and 1/2 teaspoon salt. Place half of peach mixture in a blender; process until smooth. Pour pureed peach mixture into a large bowl. Repeat procedure with remaining peach mixture. Stir in 1/4 cup onion and next 4 ingredients (through 1 tablespoon jalapeño). Cover and chill 30 minutes.
3. Arrange 1/4 cup seviche in each of 8 small shallow bowls; ladle about 1/2 cup soup around seviche. Sprinkle evenly with sea salt; garnish with cilantro sprigs, if desired.