Heat a grill pan over high heat. Shuck the ears of corn and char the corn on all sides. When cool enough to handle, slice kernels off and set aside in a bowl. Cook the shrimp. Fill a medium pot with water 3/4 full. Add the Old Bay seasoning and lemon quarters. Bring to a boil. Remove pot from heat. Add shrimp, cover and let sit for 5 minutes. Slice the avocados in half and remove the pit. Cut the avocados into small cubes. Reserve 1 cup of cubed avocado to a bowl. Squeeze the lemon over the avocado, cover and refrigerate until ready to serve. Place the remaining avocado cubes, cucumber, broth, Mexican crema, lime juice and salt in the bowl of a blender. Process until smooth and creamy. Transfer the mixture to a bowl and refrigerate for at least 1 hour or until very cold. Chop the shrimp into small pieces and refrigerate. Refrigerate the corn kernels. When ready to serve, spoon chilled soup into bowls. Top with a few avocado cubes and a few charred corn and chopped shrimp. Top with a small amount of green onions. Serve chilled.