Set a large resealable plastic bag in a bowl. Pour in the buttermilk and salt. Add the halibut fillets and close the bag. Refrigerate for at least 4 hours and up to 1 day.
Preheat the oven to 120 C / Gas 1/2.
Place half of the crispbread and half of the Italian herb seasoning into a food processor. Pulse until they become fine crumbs. Pour into a shallow bowl and repeat with remaining crispbread and Italian herb seasoning. Whisk eggs with water in a separate bowl.
Remove halibut from the bag and discard the marinade. Dip fish into the egg and then in the crumbs until evenly coated. Place coated fillets into a lightly greased 23x33cm or similar sized baking dish. Do not let them touch, as they will weld themselves together. Drizzle the butter on top of the fish.
Bake uncovered in the preheated oven until fish is opaque and flakes easily with a fork, about 45 minutes.