Combine marinade mix, water, oil and tilapia in a resealable plastic bag. Refrigerate for 15 minutes (or longer) for flavors to develop.
Wash potatoes and slice into thin slices. Place in microwave-safe bowl with 2 tablespoons water; cover. Microwave potatoes on HIGH (100% power) for 3 to 4 minutes until still firm but tender. Drain well.
Preheat grill to medium heat.
Cut aluminum foil into four 18-inch long sheets. Place 1/4 of the onion slices on each piece of foil. Top with 1/4 of the zucchini and then potatoes. Remove tilapia from marinade; discard remaining marinade. Place a tilapia fillet on top of potatoes. Fold in sides of foil to form a packet. Repeat with remaining packets.
Grill packets over Direct Medium heat for 8 to 10 minutes or until fish flakes easily and vegetables are cooked through.
Flavor variation: Substitute Brussels sprouts and leeks for the zucchini and red onions.