Bring a pot of salted water to boil. Add the scallops and shrimps, reduce the heat and poach gently for about 2 minutes.
Drain well and chill at least 30 minutes, or up to overnight.
Stir together the onion, tomatoes, lime juice, cilantro, serrano chili, and olive oil in a large bowl.
Season with salt and pepper.
Add the scallops and shrimps to the sauce and stir gently to combine.
Divide among 4 glasses or bowls.
Serve with saltines, plantain chips or corn tortilla chips.