Whisk water, oil, liquid smoke, and fajita seasoning together in a bowl and pour into a resealable plastic bag. Add avocados, bell pepper, and onion, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 1 hour to overnight; turn occasionally.

Preheat oven to 225 degrees F (110 degrees C). Stack tortillas; wrap in aluminum foil. Place in oven to warm.

Preheat grill for medium heat and lightly oil the grate. Drain marinade from vegetable mixture; transfer to a plate. Separate avocados from onions and bell peppers.

Place onions and peppers, in 1 layer, onto grill using tongs; cook until tender, 5 to 10 minutes, turning as needed. Transfer to an oven-proof pan; cover with aluminum foil. Place in the preheated oven to keep warm.

Place avocados in 1 layer onto grill; cook, turning once, until browned, about 5 minutes.

Fill tortillas with avocado and onion mixture; roll or fold tortilla around filling.