Take your shrimp, cut off the tendrils but keep the heads on. Wash them well and set aside.
Cut all your guavas in half. Using a spoon, scoop out the seeds and pulp into a bowl. Add 1 cup of water to the seeds and pulp, then mash everything together with the back of your spoon. Strain this through a fine-mesh strainer to get the guava juice. Keep this juice for later.
For your kangkong (water spinach), cut off 2 inches from the bottom of the stalks and throw these away. Cut the rest into 3-inch pieces, keeping the stalks separate from the leaves. Wash well and drain.
Get a large pot and set it over medium heat. Pour in 4 cups of water, add your quartered onion, and the halved guavas. Let it come to a boil. Once boiling, lower the heat, cover the pot, and let it simmer until the guavas become soft - about 10-12 minutes.
Pour in the guava juice you saved earlier. Add your shrimp and cook for 4-5 minutes until they turn pink. Add salt to taste.
Add the kangkong stalks first, then after 30 seconds, add the leaves. Cook just until the leaves wilt but stay bright green - about 30 seconds more.
Serve hot with rice and fish sauce on the side. For extra heat, you can add sliced chilies to your soup or fish sauce.
Remember: You'll know your shrimp is perfectly cooked when they're pink and curled into a loose 'C' shape. If they curl into a tight 'O', they're overcooked.