For the sauce: Combine the tomato sauce, ketchup, brown sugar, garlic powder, and apple cider vinegar in a small bowl. Set aside.
For the skewers: Cut scallops into halves or quarters, so that they are just slightly larger than the shrimp. Cut salmon and zucchini into chunks approximately the same size as the scallops.
Place all the vegetables and seafood onto 4 skewers, alternating their order.
Bring a grill or grill pan sprayed with nonstick spray to medium heat. Add skewers and cook until seafood begins to turn opaque on top, about 5 minutes.
Flip skewers and cook until all seafood is cooked through, 4 to 5 minutes.
Drizzle or brush sauce over skewers immediately after removing them from heat.
PER SERVING (2 skewers with sauce): 299 calories, 7g fat, 616mg sodium, 19g carbs, 2.5g fiber, 12g sugars, 40g protein