How to Make It
Light a grill and set it up for direct and indirect grilling. Oil the grate. Brush the fish with olive oil and season with salt and pepper. Grill the fish over moderately high direct heat, turning once, until cooked through and lightly charred, 10 minutes. Transfer to a plate and let cool. Refrigerate for 30 minutes. Keep the grill on.
Meanwhile, grill the poblano over moderately high direct heat and the corn over indirect heat, turning, until both vegetables are tender and the chile is charred, 8 minutes for the poblano and 12 to 15 minutes for the corn. Transfer to a cutting board and let cool slightly. Peel, stem and seed the poblano, then chop finely. Shuck the corn; cut the kernels from the cob. Let cool.
In a serving bowl, whisk the 2 tablespoons of olive oil with the lime juice. Flake the fish with a fork and add to the bowl with the poblano, corn, watermelon and cilantro. Gently toss the salad and season with salt and pepper. Let stand for 10 minutes, then garnish with plantain chips and serve.