Brush the fish with olive oil and season with salt and pepper, to taste. Grill the fish, turning once, until cooked through and opaque, 8 -10 minutes.
While fish is cooking, grill the poblano over direct heat for 8 minutes. At the same time place the corn over indirect heat, and cook for 12 - 15 minutes turning occasionally.
Peel, stem, and seed the poblano. Chop finely. Shuck the corn, and 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, to taste. Garnish with plantain chips and serve.