Beat 1 1/2 cups milk and eggs together in a large bowl using an electric mixer until frothy, about 2 minutes. Add 1 1/4 cups flour and 1/4 teaspoon salt to milk mixture; beat until incorporated, about 2 more minutes.
Heat a non-stick skillet over medium heat. Pour 1/4 cup batter into the skillet and immediately rotate the skillet until the batter evenly coats the bottom in a thin layer. Cook until the top of the crepe is no longer wet and the bottom has turned light brown, about 4 minutes. Run a spatula around the edge of the skillet to loosen; flip crepe and cook until the other side has turned light brown, about 2 more minutes.
Whisk water, 1/4 cup flour, 1/4 teaspoon salt together in a bowl.
Place butter and garlic in a non-stick skillet over low heat. Cover skillet and cook until garlic is golden brown, about 7 minutes. Add shrimp and lobster meat; stir, cover skillet, and cook until shrimp are bright pink and cooked through, about 5 minutes. Add lemon juice and cayenne pepper; stir, cover skillet, and cook for 5 more minutes.
Whisk water-flour mixture and stir into shrimp mixture. Sprinkle Parmesan cheese over shrimp mixture; cook and stir over medium heat until thickened, about 1 minute. Remove skillet from heat and spoon shrimp filling onto each crepe. Roll each crepe around filling and place in the prepared baking dish.
Preheat oven to 350 degrees F (175 degrees C).
Combine 1 cup milk, mayonnaise, and 1 tablespoon flour in a small saucepan; cook and stir over medium heat until bubbling, 2 to 3 minutes. Drizzle sauce over filled crepes.
Bake in the preheated oven until crepes are cooked through and sauce is bubbling, 25 to 30 minutes.
