Cut each avocado in half lengthwise. Remove the pit from the avocado and discard. Remove the avocado from the skin, and place the avocado flesh in a bowl. Smash avocado with a fork until desired consistency.
Add the cilantro, red onion, jalapeño, lime juice, feta cheese, watermelon, salt, and pepper to the bowl. Stir to combine. Serve immediately with tortilla chips.