Place chilis in large heat-safe bowl. Add boiling water; cover. Soak chilis 30 minutes or until softened. Remove chilis with tongs or slotted spoon and transfer to blender container, discarding soaking liquid. Add remaining 1/2 cup water, tomatoes, onion, garlic, salt and cumin; cover. Blend on high speed until smooth. Set aside.
Heat oil in large deep skillet on medium-high heat until shimmering. Carefully add chili mixture to skillet (hot oil may splatter, reduce heat slightly if needed!). Cook and stir until slightly thickened, about 10 minutes.
Stir in shrimp. Cook and stir until just pink, about 10 minutes. Serve with rice, warm tortillas or tostadas.