Step 1Shuck the corn and trim rough ends. Cut cobs into 2 or 3 chunks. Combine the coconut milk, 1 cup water, serrano chilies, green onions and 1 teaspoon sea salt in a large pot. Stir to combine, add the corn and bring to a simmer. Cover and cook, stirring occasionally, until the corn is very tender, 15 to 20 minutes.
Step 2Remove the corn from the pan using a slotted spoon, reserving the liquid, and let the corn stand until cool enough to handle. Reserve 2 pieces of corn for garnish. Scrape the kernels off the remaining cobs with a knife. Place the kernels in a blender, add the contents of the pan and puree until smooth, adding more water if needed to reach the right soupy consistency.
Step 3Strain the puree through a sieve into a clean bowl; discard the solids. Season with salt and pepper to taste and chill until very cold. Scrape the kernels off the 2 reserved cobs and chill.
Step 4Just before serving, juice half the lime over the crab in a small bowl. Squeeze a few drops from the other lime half over the avocado in a second bowl, and toss each mixture well. Squeeze the remaining lime juice into the soup and stir. Taste soup again and adjust seasoning if necessary.
Step 5To serve, ladle the soup into bowls and top with crab, avocado and the chilled corn kernels. Garnish with cilantro.