Fill a medium pot with water about 1/2 way and start heating over medium-high heat. (Pot should be big enough to fit all the corn cob halves.)
Take husks off the corn and cut the cobs in half. (It will just fit onto the pot easier.)
Once the water starts to simmer, add about a tablespoon of salt and 2 tablespoons of cilantro. Cut 3 limes in half and squeeze out the juice into the pot. Place the squeezed lime halves into the pot as well. Mix.
Add corn to the simmering water and cook until corn is done. This will take 10-12 minutes.
While corn is cooking, clean out the shrimp.
Once corn is done, use tongues to take corn cobs out and set them on a cutting board to cool.
Add shrimp to the same pot and cook until they turn pink and opaque. This will take about 2-4 minutes, depending on the size of shrimp. (If you're cooking shrimp in shells, it will take a couple of minutes longer.)
Take cooked shrimp out of the pot and let them cool.
Cut corn kernels off the cobs and add corn to the mixing bowl.
Chop cooked shrimp and add it to the mixing bowl as well.
Dice bell pepper and shallot and add them to the mixing bowl.
Add green onion, lime zest, lime juice, olive oil, some salt, and chili powder to the mixing bowl and mix everything well.
Transfer into a container with a lid and refrigerate. Keep refrigerated until ready to serve.