In a medium bowl, toss shrimp, lime zest, chili powder, and 1 tsp. salt. In a small bowl, stir lime juice, mayonnaise, and sour cream; set crema aside.

In a large stainless steel or cast-iron skillet over medium-high heat, melt 2 Tbsp. butter. Add corn in a single layer; season with ½ tsp. salt. Cook, undisturbed, 5 minutes, then add jalapeño and white scallion parts. Cook, stirring occasionally, until slightly softened, about 3 minutes. Add garlic and cook, stirring, until fragrant, about 1 minute more; season with remaining ½ tsp. salt. Add water to deglaze pan, scraping up any browned bits from bottom of skillet.

Push corn mixture to one side of skillet and add remaining 1 Tbsp. butter. Once butter is melted, add shrimp; it's okay if some shrimp overlap. Cook, turning halfway through, until pink and cooked through, 3 to 4 minutes total. Stir shrimp and corn together to combine.

Remove skillet from heat. Stir in ½ cup reserved crema. Top with cotija, cilantro, green scallion parts, and more chili powder. Serve with tortillas and remaining crema alongside.