In a blender, mix all of the marinade ingredients until smooth. Add more salt, if necessary. Pour the marinade into a plastic bag and place the steak inside. Zip the bag and marinate in the refrigerator for 2 hours.
Prep the ingredients per the instructions above.
Allow the meat to come to room temperature for at least 40 to 50 minutes before grilling. Preheat a grill on medium/high heat, with the lid closed. Brush grates with some oil. Remove any excess marinade from the meat and place it onto the grill. Grill for approximately 7 to 10 minutes, taking into account the thickness of the steak.
Place the watermelon in a blender and blend until liquid. Strain the liquid with a fine mesh sieve back into the blender. Add the lemon juice, sugar and ice and blend again until smooth. Garnish with mint.
Place the steak onto a cutting board and let it rest for 5 to 7 minutes. In the meantime, warm the tortillas on the grill.
In a bowl, mix the ingredients for the pico (except the chips). Season with salt and pepper to taste. Refrigerate until ready to use.
Slice the meat into thin strips, then cut into small, bite-sized chunks. Top with, onion, cilantro, queso fresco, salsa verde, radishes, and lime.
Serve the tacos with pico and chips, and watermelon lemonade.