Preheat air fryer to 400 degrees F for 5 minutes, and combine water and salt in a large bowl.
Cut the ends off the plantains and make a vertical slit through the skin, so you can easily remove the peel.
Cut the plantains into 1-inch thick slices.
Toss the plantain slices with 2 tsp coconut oil.
Cook in the air fryer in a single layer for 5 minutes.
Immediately remove the plantains and smash each slice with the bottom of a jar or a mug while they are hot.
Dip the flattened slices in salt water quickly and set aside.
Brush each flattened slice generously the rest of the coconut oil (3 tsp) front and back.
Place back in the air fryer for 5-8 minutes, gently flipping them halfway through, until they are golden and crispy.
Sprinkle with lime juice, if using, and sea salt.
Serve immediately alone or with your favorite dip like guacamole.