In a small bowl, mix the masa harina, warm water, and 0.125 tsp of sea salt; knead for 2 minutes until a smooth dough forms.
Divide the dough into two equal balls, press them flat between parchment paper, and cook in a dry skillet over medium heat for 1-2 minutes per side until slightly charred.
In a separate bowl, toss the shrimp with chili powder, cumin, garlic powder, black pepper, the remaining sea salt, and half of the lime juice.
Heat the olive oil in a large skillet over medium-high heat.
Add the sliced red onion and bell pepper to the skillet, sautéing for 4-5 minutes until they are tender-crisp and slightly browned.
Push the vegetables to the side and add the shrimp to the center of the skillet, cooking for 2-3 minutes until pink and opaque.
Squeeze the remaining lime juice over the mixture and garnish with fresh cilantro.
Serve the hot shrimp and vegetable mixture inside the warm homemade tortillas.