In the jar of a blender, blend the water with the spinach and salt.
Warm the oil in a large saucepan over medium heat. Add the onion and sauté until soft and translucent, about 7 minutes. Add garlic and cook for a minute more. Add the rice to the pan and let it fry until the grains are clear, stirring occasionally. Stir in the spinach water. Bring the contents of the pan to a boil. Add the epazote, then decrease the heat, cover the pan, and simmer for 15 minutes.
As soon as you decrease the heat and place the lid on the pan, melt the butter in a large skillet over medium-high heat. When the butter is bubbling but not brown, add the prawns and sauté until just cooked through, about 1 minute per side. Remove to a plate and tent with foil to keep warm.
After 15 minutes, the majority of the liquid in which the rice is cooking should have been absorbed. Remove the epazote sprig or cilantro, place the buttery shrimp on top of the rice, and cover the pan for a final 5 minutes to finish cooking. Taste the rice, and if it is already very close to cooked, re-cover the pan, turn the stove off, and let it steam for 5 minutes.
Squeeze the lime juice over the pan and serve immediately.