Prepare the ingredients as instructed above.
Preheat the grill or grill pan over medium heat.
Season the tilapia with cumin, cayenne, salt, and black pepper. Drizzle with olive oil.
Gril the corn on all sides until nicely charred and tender. Spread with butter and season with salt and pepper.
Grill the fish to an internal temperature of 140°F (71°C). Cut into pieces.
Prepare tacos: Warm taco shells on the grill if desired.
Make sparkling chia fresca: mix all the ingredients until combined. Let sit 5 minutes for the chia to bloom.
Assemble and serve: Fill taco shells with fish, radish, cilantro and lime. Serve with grilled corn, watermelon slices, and sparkling chia fresca.