In a medium mixing bowl, combine the watermelon, onion, green bell pepper and fresh herbs, if using, with the lime juice and zest. Season with sea salt and pepper, to taste; set aside.
Heat 1 tablespoon olive oil in a large sauté pan over a medium flame until hot. Lightly season the cod with salt and add to the pan. Cook for 8-12 minutes, flipping halfway through, until fish is completely opaque. Break up slightly with a spoon or spatula and set aside.
While the fish is cooking, heat the tortillas, 1-2 at a time, for a few seconds in a large dry sauté pan over a low flame, flipping each once. Remove to a plate and cover with a dish towel to keep warm.
Use 2 tortillas for each taco to avoid them breaking apart. For each taco, spoon 1/4 of the cod onto two stacked tortillas. Top with watermelon salsa and another squeeze of lime, if desired. Repeat with the remaining tortillas.