Cut each zucchini in half lengthwise. Using a spoon, scoop out the center flesh to create a shallow well in each half, leaving about a 1/4-inch border of flesh around the edges and bottom. Roughly chop the scooped-out zucchini flesh and set it aside for the filling.
Brush the zucchini boats lightly with olive oil and sprinkle with salt. Set them aside while you prepare the filling.
In a skillet over medium-high heat, cook the Italian sausage, breaking it apart with a wooden spoon, until browned and cooked through, about 5 to 6 minutes. Add the onion, garlic, and chopped reserved zucchini flesh. Cook for another 3 minutes until softened.Stir in the marinara sauce, sun-dried tomatoes, Italian seasoning, salt, and pepper. Cook for 1 to 2 minutes, then remove from heat.
Preheat the air fryer to 375 degrees F. Spoon the filling generously into each zucchini boat, mounding it slightly.
Place the stuffed zucchini boats in the air fryer basket in a single layer. Cook for 10 minutes until the zucchini is tender but still holds its shape.
Sprinkle the shredded mozzarella on top of each boat. Cook for 2 to 3 more minutes until the cheese is melted and bubbly with golden spots.
Carefully remove the zucchini boats from the air fryer. Top with fresh basil and let them cool for a couple of minutes before serving.