Whisk water, olive oil, liquid smoke, and fajita seasoning together in a bowl. Pour into a resealable plastic bag, then add avocados, bell peppers, and onion. Coat thoroughly with the marinade, remove excess air, and seal the bag. Marinate in the refrigerator for 1 hour to overnight.

Preheat oven to 225°F (110°C). Stack tortillas, wrap them in aluminum foil, and warm them in the oven.

Preheat grill to medium heat and lightly oil the grate. Drain marinade from vegetables. Separate avocados from the onion and bell peppers.

Grill onion and bell peppers in a single layer. Cook until tender, about 5-10 minutes, turning as necessary. Transfer to an oven-proof pan and keep warm in the oven.

Grill avocados in a single layer, turning once, until browned, about 5 minutes.