Adjust oven rack to lower-middle position and heat oven to 400ºF. Dissolve 1½ tablespoons salt in 2 quarts piping hot tap water in a 13-by-9-inch baking dish. Add noodles and soak until soft, about 10 minutes. Drain in a colander.

Meanwhile, cream together 8 ounces of the cream cheese with ¼ cup of the broth; gently fold in spinach, shrimp, and crab meat. Mix remaining 4 ounces cream cheese with remaining ¼ cup broth in a small bowl; set aside.

To assemble, smear ¼ cup marinara sauce over bottom of baking dish, then assemble 4 layers of each of the following: 3 lasagna noodles, a scant cup marinara, 1 cup filling mixture, ¾ cup mozzarella, 2 tablespoons Parmesan.

Top with remaining 3 noodles, cream cheese-broth mixture, remaining 1 cup mozzarella, and remaining ¼ cup Parmesan.

Spray one side of a large sheet of heavy-duty foil with vegetable-oil cooking spray. Cover baking dish with foil, oiled side down, and bake until bubbly throughout, 40 to 45 minutes. Leaving lasagna on same rack, turn oven to broil. Remove foil and broil until lasagna is spotty brown, 4 to 5 minutes. Remove from oven and let sit for 10 minutes. Cut into squares and serve.