To make the Teriyaki Sauce, combine the soy sauce, maple syrup, pineapple juice, garlic, and ginger in a small saucepot. Place it over medium heat. In a small bowl, make a slurry with the water and cornstarch, whisking together until the cornstarch is completely dissolved. Slowly pour the cornstarch mixture into the saucepan, stirring continuously. Bring to a boil and boil for 2 minutes, until the sauce begins to thicken. Remove it from heat and set aside.
Cut the tofu into 1x1" cubes. Place in a shallow container. Pour the Teriyaki Sauce over the tofu so that the tofu is completely submerged. Place in the refrigerator and let marinate for 1 hour. Reserve the marinade when the tofu is done.
Preheat the oven to 450F. Line a baking sheet with foil and set aside.
Fill your skewers with tofu, onions, bell peppers, and pineapple. There should be enough tofu for 8 skewers if you place 3 pieces of tofu per skewer. Bake for 25-30 minutes, flipping every 10 minutes until edges are charred and sauce is sizzling. Remove from the oven. Brush with reserved marinade and serve immediately.