Make the custards: Place the water in a small bowl. Sprinkle the gelatin over the water and set aside until the gelatin is softened, 5 minutes.
Place the cream, milk, 3 tablespoons of the sugar, and the salt in a saucepan. With a small knife, cut halfway through the vanilla bean lengthwise. Open the vanilla bean and scrape the seeds into the pot. Add the beans to the vanilla mixture. Bring to a boil over high heat, reduce to medium and boil for 1 minute, stirring constantly. Watch closely so it doesn't boil over. Remove from the heat and whisk in the remaining 1 tablespoon sugar and the gelatin mixture until dissolved.
Strain the cream through a fine-mesh strainer; discard the vanilla beans. Chill in the refrigerator or over an ice bath just until cool and slightly thicker than heavy cream, but not set. Pour into 6 5-ounce ramekins. Refrigerate for 3 hours.
Make the sauce: Puree 1 1/4 cups raspberries in a blender with the water, sugar, kirsch, and lemon juice. Strain through a fine-mesh strainer. Place in a bowl and add the remaining 3/4 cup raspberries; stir together.
Just before serving, run a small knife around the edges of the ramekins. Unmold the panna cotta onto serving plates and spoon the sauce around the custards.