Preheat the oven to 425 degrees F (220 degrees C).

Bring water and crab boil to a boil in a large saucepan.

Stir in lobster tails and boil until they turn bright red and the meat becomes opaque, 5 to 8 minutes. Drain and cool lobster tails. Remove shells, and dice lobster meat. Transfer to a bowl.

Place a pie crust into the bottom of a 9-inch pie pan and set aside.

Heat butter in a skillet over medium heat. Cook and stir onion and celery in butter until tender, 5 to 8 minutes. Stir flour into onion and celery mixture until vegetables are coated.

Combine chicken broth and milk in a bowl. Slowly stir broth and milk into onion, celery, and flour mixture until thickened. Stir seafood seasoning, garlic powder, and black pepper into thickened sauce.

Mix thawed vegetables, diced potato, and cooked lobster meat into the sauce; simmer until potato is tender, about 8 minutes.

Pour lobster filling into the prepared pie crust.

Cover with remaining pie crust and seal by crimping the edges of the two crusts together. Cut an X into top of pie crust with a sharp knife to allow steam to escape during baking.

Bake pie in the preheated oven until crust is golden brown, 40 to 45 minutes.