Juice pineapple in a juicer, or peel and core the fruit, then purée in a blender and strain. Pour juice into a medium bowl.
Stir in red bell pepper, cucumber, red onion, serrano chile, and chopped cilantro. Season to taste with fresh lime juice and sea salt.
Cover and chill until very cold, about 2 hours. Serve garnished with cilantro leaves.