Mix the steak with the rest of the marinade ingredients and allow to sit in the fridge for an hour and a half or as long as overnight.
Grill the corn on high heat to char, remove from heat. Put the steak on the grill next and cook to medium doneness. Cook the zucchini slics as well to char.
While the steak and zucchini cooks, cut the corn off the cob. Mix it with the watermelon, scallions, cotija, garlic, and lime.
Cut any chunks of steak that are too big to fit in the taco shells so you end up with 12 equal portions of steak. Ideally when you cut the meat make sure to go against the grain.
Build the tacos starting with the steak, then adding a zucchini spear, then the salsa.
Serve immediately.