Arrange the shrimp on a large platter. Sprinkle with the minced jalapeno and the chopped cilantro. Pour the lime juice over and let marinate for at least 30 minutes, refrigerated. When ready to cook, season lightly with salt.
In a small bowl, sift together the flour and baking powder. Whisk in the water until the batter is smooth, and then add the remaining ingredients, seasoning with cayenne and salt to taste. If made earlier in the day, set the bowl into an ice bath.
At serving time, in a heavy saucepan, wok, or deep-fryer, heat 3 inches of oil to 350 degrees F. Dip each shrimp into the batter and coat well. Carefully place 1 or 2 in the hot oil and cook until golden brown, about 2 minutes. Remove with a slotted spoon or a flat strainer and drain on paper or clean toweling. Keep warm while preparing the remaining shrimp and the spinach.
Arrange a few fried spinach leaves on half of each of 4 plates. Place 4 shrimps on the other half and serve immediately.
In a wok or deep, heavy saucepan, heat about 3 inches of peanut oil to 375 degrees F. Clean the spinach leaves well, trim and dry thoroughly. Fry until crisp and translucent, about 2 minutes. Remove with a slotted spoon and drain on clean toweling. Season with salt to taste and serve immediately.