In a 2- to 3-quart pan over high heat, bring 4 cups water to a boil. Add scallops and shrimp, cover pan, and remove from heat. Let stand until scallops and shrimp are opaque in thickest part (cut to test), about 5 minutes. Drain shellfish and pour into a bowl; nest bowl in ice water. Stir occasionally.
Meanwhile, coarsely chop tomatillos. In a blender or food processor, whirl tomatillos, lime juice, and cilantro until puréed. Pour over scallops and shrimp. Add onion and bell pepper. Mix occasionally until ceviche is lukewarm or cool, 20 to 30 minutes.
Add rice; mix and season to taste with salt and hot sauce.
Arrange spinach on dinner plates and top with ceviche mixture.