Set oven rack about 6 inches from the heat source and preheat the oven's broiler. Line a baking sheet with aluminum foil. Place peppers with cut sides down onto the prepared baking sheet.
Cook under the preheated broiler until the skin of the peppers has blackened and blistered, 5 to 8 minutes. Place blackened peppers into a bowl and cover tightly with plastic wrap. Allow peppers to steam as they cool, about 20 minutes. Remove and discard skins.
Combine roasted peppers, canola oil, pepitas, red wine vinegar, lime juice, cotija cheese, garlic, salt, and pepper in a blender or food processor. Blend to combine, about 10 seconds. Add cilantro a little at a time and blend until smooth.
Combine mayonnaise and water in a large bowl and whisk until smooth. Add the blended pepper mixture to the mayonnaise mixture and mix thoroughly.